AFAIK NVIDIA and AMD do this unasked for popular game releases because it gives them a competitive advantage if 'popular game X' runs better on NVIDIA than AMD (and vice versa). If you're an AAA studio you typically also have a 'technical liason' at the GPU vendors though.
It's basically an arms race. This is also the reason why graphics drivers for Windows are so frigging big (also AFAIK).
I think this is very accurate. The exception is probably those block buster games. Those probably get direct consultancy from NVIDIA during the development to make them NVIDIA-ready from day 1.
It's basically an arms race. This is also the reason why graphics drivers for Windows are so frigging big (also AFAIK).