Frank Oz hasn't really quit, however, the only Muppets he manipulated so far are Cookie Monster and Grover. He still does Cookie in the "Letter Of the Day" inserts on SS. I don't think he came back as Bert, Fozzie, Piggy, Sam or Animal. He's cut back on his muppeteering so he can be seen as a serious director. Here's what I read. Oz left the team when Henson died because he could not stand to be there without his long-time partner and friend. Yet he still came back to do Bert, Cookie Monster, and Grover. During that time, he stated he would never perform Bert to someone else's Ernie. Maybe that's why he's semi retired. It must be too painful without Jim.