Actually, they do all the time. They even convert sculpts to textures quite often.
Modeling stuff in high poly sculpture programs like mudbox is not something that is game industry standard.
Explanation;
People start out with high poly models and lower the poly count for various games all the time, ....
I don't know 1 game company that does that. It's a silly way to approach game models.
Do they use sculpts to generate UV? Yes. Do they convert sculpts to models for game use? No. That's what i'm talking about here.
I have two acquaintances that have worked on various games. (Stephon worked on a lot of DS titles and my friend Christopher works for blizzard down the street in irvine.)
I have seen both of them model, and in no way does anyone else in the industry go as ass backwards as sculpting a model in such detail that their engine can't render it. or it doesn't meet their poly count goal.
I have seen sculpting to create UV and textures. Yes. But NEVER converted a sculpt to a in-game mesh.