Because, like it or not, the majority of people's education is the means to obtain a job in a related field. People don't go tens of thousands of dollars into debt for some l'art pour l'art interest development; they do it because they expect the benefit from the degree (i.e., as a ticket into the job market in a field related to the degree) to outweigh the cost.
The community college system is more than enough for those who view postsecondary education as a mere vocational program and doesn't require going "tens of thousands of dollars into debt".
I'm not arguing for the status quo, but let's be honest about the current state of higher education in America: a ton of people have too much debt because of the pressure to attend a 4 year university. This pressure is usually of the form, "good education => good job". Moreover, "good" is often conflated with "expensive" or "prestigious" with regards to education, and conflated with "high-paying" with regards to a job.