Mark Walberg, Will Smith, and Morgan Freeman worked in Hollywood for many years before becoming stars.
It's tough to make it big in Hollywood. Disney or not, even if you manage to get some name recognition with a TV show, it can be very, very difficult to turn that into a successful career. For these "tween" stars in particular, it can be very, very hard to shake the squeaky clean tween image and get taken seriously as an adult.
Some teen and tween actors who were popular when I was a kid: Devon Sawa. Jonathan Taylor Thomas. Macauley Culkin. Neve Campbell. Claire Danes. Luke Perry. Jason Priestly. Joshua Jackson. Jared Leto. Leonardo DiCaprio. Jennifer Love Hewitt. Freddie Prinze Jr. Tiffany Amber Thiessen.
I could go on. But really, Leo is the only one who has been able to maintain his status. Surely it helped that he happened to star in the most successful movie of all time. I'm guessing you haven't heard of most of the rest - some have managed to have a successful career as a working actor, even if the aren't superstars. The rest have completely dropped off the map. It's much worse for female actors, because once they start to age a bit, the number of possible roles drops tremendously.