There's no doubt that colleges which make you live on-campus are forcing you to raise your cost of food & board, but that doesn't seem like it's the majority position. Estimates range from 1/3 of all college students living on campus (Department of Education Firewatch Publication, 2009) to even lower:
National Retail Federation (NRF) 2007 Back-to-College Consumer Intentions and Actions Survey wrote:When it comes to a student’s living situation, half of those polled (49.7%) said they would be living at home during the school year, while more than one-fourth (28.6%) will live in off-campus housing, one-fifth (18.7%) will stay in a dormitory or other type of college housing and a small fraction (1.3%) will live in a fraternity or sorority house.
Per that, at least half of all college students do exactly what you'd say they should do: live at home and exploit economies of scale.
Further, the US Census found that out of an estimated 16-18 million full-time enrolled college students, 2.2 million lived in on-campus housing. So, by that accounting, on-campus housed students account for 12.2-13.7% of all full-time students. Including part-time students makes that number shrink even further.