Also those drivers are huge (really, HUGE) and have incorporated pieces of code with widely differing IPs, patents previous companies and other sources for which opensourcing would probably be a major legal hassle and lawsuit risk.

Also... if you're nVidia / ATi... what's the gain in giving optimization paths and software optimization database to the competitor? Those drivers are full of application / game specific optimizations to make them look better / run faster / workaround bugs.