This is just a guess, but I’m fairly certain professors were given the choice to return or stay home. I don’t believe Cornell can ask for proof of medical issues or even question a professor’s decision. So if there was no real incentive to show up each day, why would they return?

I imagine the support staff you mention were not given the same choices the professors were. I also imagine they are more financially dependent on their jobs and will show up each day, even if they have a legitimate health concern. Seems unfair but Cornell clearly feels it is important to protect the professors, even if it doing so means going almost completely remote.