The issue is one of licensing, not technology. There’s all kinds of patents in the space, and using free codecs could still infringe them. DirectX doesn’t have the same patent protection. I believe in theory you could make a fully open source Linux native version of DirectX.
For more info from someone who knows more than me, see here.
Noveau is terrible for gaming. If you want any kind of reasonable gaming experience you’ll need the propietary Nvidia driver (for now).