These people are the only family that Alex has ever known. I'm not sure she would abandon all she has ever known so easily. It would take a lot of balls for her to run off into a jungle full of bears and security system monsters and of course, I'm sure she knows they will eventually find her. Maybe that is why Carl was locked up. He keeps running and keeps getting caught. And she can see by how they treat him that it is better to just stay put.