Windows games are generally not made in dotnet, and games which are made in dotnet (mostly Unity) are actually run on Mono. AMD's Mesa RadeonSI(free) driver frequently outpaces their fglrx(proprietary) driver on recent hardware. NVIDIA and AMD distribute good quality proprietary drivers for Linux, but given the pace the Linux kernel takes, and the benefits of integrating with the native interfaces of the kernel, AMD has decided to write good quality open source drivers as well as their proprietary ones. NVIDIA blocks community open source efforts by delaying the release of firmware images vital to enabling basic features of their GPUs. SteamOS is shipping fglrx for now, and fglrx doesn't currently rely on the DC kernel code. Most SteamMachines seem to be NVIDIA-based for now, and while it's inconvenient to ship their proprietary driver, it is in no danger of failing to work.

Read an article or something.