Not only do the riders not have anything to gain and everything to lose as eon said, but their sponsors wouldn’t have it. Even if Valentino, Hayden, Edwards, Pedrosa, etc. WANTED to compete, they’d get forced out by their management and sponsorship. This short blurb kind of alludes to that:
Notice: not riding his GP YZF-M1, but rather a production YZF-R1 (saves $1+ million in potential damages), and riding with old geezer race hero of old behind a pace car.