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.