There is an environment map for each sky. And as almost every track has it's own sky, you can also make environment maps for the specific track. If it's not so hard, why don't you do it?
The problem is, that if you put e.g. skyscrapers on the South City environment map it will look silly, because the skyscrapers on the car won't move. It's the same for other objects, like trees on Blackwood. That's why all original environment maps don't have such large objects on it, so you cannot see that it's not real dynamic environment mapping (which is technically possible, for example GP4 from 2002 already had it) but just a static image.