College students have to be somewhere. They don't magically disappear when schools are closed. So this just means they will be getting infected and transmitting the virus in other places. How is that better?
Navigating dorms often requires close contact (elevators, dining halls, shared bathrooms, shared rooms). Social gatherings in near-campus bars, fraternities, and on campus parties will invite infection, as party events at various summer hotspots have demonstrated already. Classrooms are probably the easiest venues of campus infection to mitigate.
The virus doesn't spread at a constant rate just because people exist.
An infected person comes into contact with other people to a sufficient degree to transmit the virus. Those people, in turn come into contact still other people to a sufficient degree to transmit the virus to them, etc.
At a school, groups of students form, both officially (e.g., in classes) and unofficially (socially), often in ways that are friendly for transmitting the virus. Then groups reform (students go to their next class, or meet with another group of friends or attend another activity), then reform again... and again... and again, etc.
Of course, this doesn't entirely go away when students are off campus. But the transmission vectors are so much smaller -- classes and groups for. other organizations don't occur; social groups tend to be smaller, more stable and with less overlap into other groups.
