I have mixed feelings about the whole thing, so it might be a stretch to say I support requiring masks on planes. But, I think the reason for it is that for the people who want masks there's little alternative. It's not like a restaurant or a store that you just can't go to or choose another store to shop at. You're stuck on the plane. A mask mandate will also encourage people who are uneasy of flying to get on planes. I don't really care for myself, but my stepmother who is immunosuppressed and elderly is flying to my niece's graduation next month. Not feeling great about her breathing canned air with a bunch of others. And I know of more than one person who's gotten on a plane with Covid.
Honestly, I just don't understand the opposition to it. It's not a big deal. Hell, i wore a mask on a 12 hour overseas flight with a connection---so with all the airport time well over 15 hours, and it was mildly inconvenient at most. And after years of catching colds every time I flew, that hasn't happened at all post-pandemic.