it's true and I hate it.
Most of my favorite games don't have a serious storyline, like Tetris Attack, Geometry Wars, even Team Fortress 2 or Multiwinia.
Many games these days absolutely must have some kind of storyline, and to me it's really depressing. What happened to turning the game on to just have fun? Why must we be moved by video games storylines? Why not the game play itself?
EDIT:
Post about your favorite games that don't have storylines, and what games have unnecessary storylines.

Storylines are integral to non-casual games only. It has always been this way, going all the way back to the beginnings of video games. Casual games, like the ones you list, have always been about gameplay and gameplay alone, this really has not changed at all in modern gaming (just look at games like Bejeweled, Luxor, SSBB, all the ones you listed, etc.).
As for non-story games, this is kind of embarrassing, I've always been partial to the Super Monkey Ball games.

Many people would not consider Team Fortress 2 or Multiwinia to be casual games.

Counter Strike is a good example. Unless one can count terrorist vs counter-terrorist as a story. ;)

Well, you certainly do have to draw a line somewhere, this is why I did not include Left 4 Dead in my post, because even though it's a multiplayer game, each level is supposed to be a story in and of itself.

Grand strategy games typically do not have a storyline. In games like Civilization I-IV, Imperialism, and the Total War games, the story is just the way the game unfolds as you play it. The conflicts that arise are the result of the conditions on the map rather than an imposed narrative. Even some smaller-scale strategy games like Tropico or the SimCity series don't have a set storyline.
One strategy game that did have a good story was Alpha Centauri. The story that went along with the Transcendence victory was not only interesting in itself, but well-integrated into the gameplay mechanics. It made the map another of the factions in the game.
RPGs and adventure games need storylines by definition, at least in my mind. Killing things and taking their stuff isn't interesting to me in itself. It needs some purpose. And an adventure game without a story would be just a simulation of a regular day: talking to people about random things and wandering around without any destination.
My favorite game without a storylilne is probably Civilization IV.
A game with an unnecessary storyline... for some reason Myth is what comes to mind.

If you look at the strict definition of a casual game, i.e. a video game or online game targeted at a mass audience with no specific type of gameplay, fits into pretty much any genre, has relatively simple rules and lacks any kind of real commitment to play, pretty much all those games could be considered "casual".

What about stuff like sims & spore? They're considered AAA (for some reason) yet lack stories
Civilization has no story
Racing games...
Sports games...
Personally I think games need stories but they need to be skippable so if you've finished it and all you do want is to load it up for a quick round of monster shooting then you can.

I would disagree, in order for you to be good competitively at something like Tetris Attack (Puzzle League if you will) You need to have a high commitment to playing the game. Same goes for Team Fortress 2 and Multiwinia. I could pick up Mass Effect for a few minutes and then set it down, does that make Mass Effect a casual game? I don't quite think so. I don't like to think in terms of "casual" or "hardcore" I like to think of whether or not it's fun and warrants more than one play through. Games like Max Payne, typically only require one play through because the reason for playing is the storyline. Afterwards you might play for the gameplay, because of the higher difficulty levels, but for most people one play through is enough. While a game like Tetris Attack would be a game people would return to frequently to play again and again. Same goes for games like Rockband, or Bomberman.

Just had some great fun at a friends playing Burnout: Paradise. No story to speak of, but great fun.

My favorite game without a storyline is Ultimate Doom.
I'm pretty sure you all have to agree with me that Doom has as much story in it as any action movie from the 80's, that is none.

Depends on what you mean by triple A. Popcap spends tons of money on their new games and they very rarely have stories...and bring in tons of money.
Just saying ;)

Even if games have storyline, I rarely play them for it.
I just finished Call of Duty: World at War, and I didn't take any notice to the storyline (Well, it was a WW2 FPS, which means copy/paste with new graphics and mechanics), but the gameplay was beyond awesome. High action, surprisingly amazing soundtrack, and just plain gory fun.
I'm also playing Painkiller atm, which is insanely fun, but has practically no storyline as well. Something about the generals of hell, or whatever, but who cares? To quote mister Croshaw: "SHURIKENS AND LIGHTNING!"
I think the only games I really care about the storyline in, are RPG's and certain RTS's, and some of the horror adventure games I've been getting into lately (Penumbra Black Plague and Call of Chtulu Dark Corners of the Earth).

The first game that kinds for me is Tony Hawk's Pro Skater... a series that was basically ruined with the introduction of a "story".
Early THPS games were arcade games with a small exploration aspect. There wasn't much to do, but what little gameplay there was was perfectly balanced.
Later games added a story mode... a small change that I like to think was made by some marketing manager who had absolutely no idea what he was doing: It looked like a small change that added countless possibilities for marketing, like including famous skaters and stuff like that.
But what nobody expected was how much it would change the nature of the game: Since events were now connected, they now had to take place in an open area so that the progression from event to event would make sense. And in that the old arenas had to go and be replaced with "hotspots" that while technically part of the open world were also separate areas in which tournaments could take place. Needless to say that these hotspots became a major annoyance because they were open so that lots of rails, streets and jumps would accidentally cause you to leave that area. Then there was the traveling and exploration that was now needed to do what was previously a simple menu selection. Instead of just jumping right into the game you now had to skate around with nothing to do but find your next task. And lets not forget about the linearity that was enforced by the new system. While you still could return to previous unfinished challenges, this now felt strange because you now got introductions to each challenge that was tied to your current progress. When you returned to an unfinished challenge you would get an introduction that wouldn't fit whatever you had achieved in the meantime anymore, making it feel completely unnatural. And of course, there are the fillers that had to be introduced to make the boring parts of the large maps seem at least a bit interesting. Stuff like "repeat the moves of XY". Urgh.
I love a good story/scenario as much as everybody, but why every game should need one is something I'll never understand.
But it isn't just that every big title now gets a story that it doesn't need... I'm also seeing less and less of the typical non-story games on modern consoles
Where's my SSX? Where's my Smuggler's Run?

Storylines are integral to non-casual games only.
I wouldn't agree with you. A large number of so-called casual games are heavily based around storylines these days. Although not apparent in the gameplay. It's rather hard to tell a story during an actual hidden object hunt, for example (although some newer games do attempt to do this).
Likewise with the other staples for the casual label, Match 3, Time management, and so forth. They are all seeing an increase of storylines encroaching onto the gameplay or being used to further indicate progress in a game during level transitions.
And ultimately, any game which you can pick up for only a short while and enjoy, is just as much a "casual" game as it could be considered a "hard core" game. Such as Saints Row 2. You can pick it up, do something within it for 5 to 10 minutes and then put it down having gained some enjoyment and possibly, some progress.
Same goes for Burnout Paradise (an absolute gem of a gem). You can just pick it up, choose to run a race, find a jump, or drive for the sheer fun of it, then put it down 10 minutes later.
However, TF2, CS:S and so forth are not what I would call a "casual" game, as they rely on other people. You can't really just pick it up and then put it down as you have to consider the other players too.
But to be honest, I hate the terms "casual game" and "hard core game". A game is a game. They are all designed for one primary purpose; to have fun. It just seems that some people have forgotten this, sadly, that includes developers too.