For various reasons. Many unis are close to bankruptcy even without this pandemic. If they lose a large number of on campus students, then it'll be tits up. It would be no surprise if some opponents won't have the means to open their doors if everything is forced to go on line.
Second, would advertisers be willing to continue to sponsor games that few people attend? I don't think so, especially local advertisers that network affiliates are deeply dependent. Lots of people losing jobs, clients, business. How are those folks going to have the capacity to attend games? Attendance has been falling for several years, this will greatly accelerate the trend.
Third, it seems we're just getting started with the pandemic and resulting economic depression. NYC is on total lockdown, Los Angeles is seemingly a few days away from that situation as well. Don't give me itsjustdaflubro bullshit. The most important American city doesn't go on full quarantine because of a bad case of the flu. Something else is going on, and it would be no surprise if most unis/colleges don't open next fall. Football won't mean much under those circumstances...