hello. that some one know why topher grace dosen't stay after the last episode? everyone stay but he dosent wait he ran away...why?

Well, he pursued a movie career during the last season, that's why he didn't appear in the final season. But when he returned, he stayed, he was just uncredited, more like a returning icon as they called, not so much a special guest appearence like Ashton Kutcher in the first 4 episodes of the season.