The lack of racing adult games might be due to the extra amount of work required for the racing part, especially considering I didn't see that many plugins for racing games and ai on the marketplace, so I had to make mine from scratch. Also, the vast majority of adult games seem to be just visual novels.
In the patreon release for Exoterra there is a toggle to make Kaitlin busty, chubby, buff or skinny.
GTA San Andreas also had a mechanic where CJ would gain weight when eating too much, get buff when training a lot etc.