Because they fall under tons of pressure. They're stuck upholding these wholesome G rated personalities and lives and are not allowed to make mistakes in the real world. Children and parents alike kind of shove these boys and girls into the roles they're known for and that's It.

On top of working so much on whatever show they're on, plus maybe recording albums, appearances, plus schoolwork... It's a lot for a young person- even if they realize That it is and understand what they've gotten into.

So when they get older, they want to break out of that picture that the network and the General public have painted them in.

It happens with Nickelodeon stars too.