@David09 - I think some directors just love acting and never want to lose touch with their craft. It’s not uncommon to see actors eventually wind up as directors. When they do that they go different paths.
Some of them just want to stay out of the acting spotlight permanently after that, while others like to dip their feet in the water now and then. I think it just depends on the director.
I saw Clint Eastwood play a role in a movie he directed, El Torino. If you ask me, I think Eastwood never wanted to get away from the craft of acting. I think he felt as long as he had some acting role, whether major or minor in a film, he would understand better how to direct. I don’t know if his reprise in El Torino would be considered a cameo appearance, however, since it was an important role.