In short:
1. Because it is ineffective - the games get pirated anyway, usually within hours of release.
2. It is inconvenient, and often interferes with the game
3. People who DO pay for the game resent the fact that pirates have the convenience of not having DRM.
Take the 'constant internet connection' DRM. When the server goes down, you can't play the game at all (even as single player). Not true with a pirated version. Or take the popular "only so many installs" DRM. If you change your hardware a couple of times your game is no longer valid, or you need to go through a lengthy support processes with the game publisher. Not true with the pirated version.
If DRM actually stopped people pirating games, then there could be an argument in favour of it. However it does not. To my knowledge, every single game released with DRM has been pirated.
Assassin's Creed 2. There's a reason EA jumped straight into the 'always on' DRM idea after that came out.
Plus, Steam is essentially 'always on' DRM. You can have offline mode, yes, but you still need connection to download the games or update them, it's merely a more sophisticated always-on system.


PC naturally will have the best gfx/controllers 
If you don't know how to download pirated games, then you will be buying originals anyway