The same people who don't want to be pigeon holed as Batman, Superman, Bourne etc.
RDJ is an exception though, I think he has the acting chops to do Iron Man as long as he wants and still not be pigeon holed.
Matt Damon has done so much other stuff that I don't think he has anything to fear. Christian Bale has done a bunch of other stuff too, but Batman is such a strong character that he will always be associated with it. There hasn't been a memorable Superman since Christopher Reeves.