Running a college/university campus is an expensive operation. Many expenses go unrecognized, such as employing campus workers like safety officers, janitors, administrators, etc, and maintaining campus facilities.
These are fixed expenses that mostly get paid for by student tuition and fees. If students don't return to campus, the universities will have to use their endowments – which is like a college's savings account – to pay these expenses.
Colleges depend on a steady and predictable flow of money from students every year to run their operations. Out of state and international students are especially important because they pay more and will likely be the ones least likely to return. In addition, colleges make a lot of money from athletics and conferences held on the campus.
The consequence of fewer students returning to campus is that the cost may have to be absorbed by the college itself, causing them to run into a deficit. Endowments supply scholarships and other financial assistance programs, and so if the college is faced with a more expensive operation there will be less money available to help students who are struggling financially. Another result of these financial issues is that many colleges are having to layoff professors in order to cut expenses.
In addition, campuses that do reopen will face even higher running costs as they roll out expansive testing programs which aim to test students up to three times a week.
Colleges have also had to renovate various campus spaces to comply with COVID safety guidelines, and are stocking up on expensive protective equipment.
Altogether, running a campus is an expensive operation and without student room and board fees, colleges are going to struggle to stay financially afloat.