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As one of the few people I see who still wears a mask on airplanes, I think back and wonder how this wasn't always a thing, even before COVID.
Travelers are in a closed space for a prolonged period of time sharing possibly-infected air with people who have been all around the world and likely in close company with even more people.
COVID or not, wearing a mask is only a minor inconvenience and it shouldn't even be a question about whether or not to wear one, provided you care about your own health and/ or the health of others.
Reply-guys, please don’t give me your dumb "BuT tHe AiR iS FiLtErEd" excuses. It's still contaminated air before it gets recycled/filtered, and people are still breathing that in and out. It doesn't matter if it gets filtered AFTER that has happened.
@Alice The worst cold and the worst flu I ever got were both contracted in pre-COVID trips that involved air travel. Never not masking on a plane again.
@maxleibman @Alice
Two days after arriving at relatives for Christmas, I woke unable to talk - viral laryngitis. Woman in my row on airplane was NOT well but flew anyway. I finally asked airline staff to move me to another seat. Just didn't do it soon enough. Or maybe it wouldn't have helped anyway. This was 2 yrs before Covid. Spent my visit in the guest room- was better about 2 days before flying home. 
Masks are life savers.