Why do you think that talented actors are willing to put their time and effort into bad movies when they are talented enough to do more?
Money.
Had to chime in on this for a sec.
There's a quite wonderful extra on the DVD for
Runaway Jury featuring a conversation with Dustin Hoffman and Gene Hackman.
In it, the two men basically say that every movie they make they wonder if they'll ever work again; if the roles will finally just dry up completely and their agents will stop calling.
That was eye-opening.
My visceral response (and I hope many other peoples') was:
Holy shit, you're Dustin Hoffman and Gene Hackman!! You helped define cinema for a couple of generations! Why aren't there people beating down your doors with telephone-book like piles of scripts and briefcases full of money?!
Sometimes, well, talent isn't enough. And acting in movies is a job; like many other jobs, you have to keep working, or the people looking at your resume start to wonder why you haven't been, and why they should take the chance of being the one to break the pattern.