Is SECDRV.SYS installed as a dummy file because they couldn't completely remove it from the game?

Probably. There's a non-zero number of games that used it, and they tended to hook it deep. This is part of why SecuRom got sued into Morrowind and then disabled in later versions of Windows.

This would be easier than actually hacking or recompiling the executables to remove it.

Yep, it's a dummy. A lot of newer games here also have a steam.dll in the install directory because ripping out the DRM system completely is much more complicated than just replacing it with a dummy that auto-completes all DRM checks locally.

Thanks for the responses.