Because then they’d have people going through their entire adult lives in a college, being supplemented by their parents and the government, constantly taking classes and not being efficient in choosing a discipline, thus wasting the taxpayers’ and government’s time and money.
It’s nice to be able to learn new things every day, and take courses, and not be responsible nor worry about how to pay for your expenses because someone else is dealing with that. However, you would then have a problem, in that, the colleges would be extremely large with hundreds of thousands of students who wouldn’t move on or launch. There is a limit to how much the state, or others, can and will shell out for one individual student.