There are just so many things that can go wrong.
History.
Way back in Win NT 4.0 they, (Microsoft), decided to move the GDI (Graphic Device Interface) in to the Kernel space (Ring 0) for performance reasons. When the GDI wasn't in the Kernel space it wouldn't crash the complete system. So you might lose the display but the server would keep running. And you could reboot the server at a more convenient time via other means.
Now, however, because the GDI is in the kernel, if the developer messes up in writing the KMD (Kernel Mode Driver) it brings the whole system down. BTW writing a solid it no trival task. It takes years of work to get it right.
Some examples:
A User has video driver Ver. 1 on their computer, but the game developers tested their game with the new video driver Ver. 2, which has some fix(s) in it. So when the user play's the game, Bingo! crash!
Also a (imaginary in this example) fix might only relate to users with 2 Gb of RAM or more. So the majority of users will never have a problem, BUT, as time goes by, technology changes and the user adds RAM. So now the game access RAM above the 2Gb range, Bingo! crash. The user is left sctratching their head, thinking, I just added RAM and now my super cool game crashes? It should be running better?