Thanos can NOT fly. He has never flown, nobody has seem him fly, I haven't seen him fly, you haven't seen him fly, his bios don't mention one thing about being able to fly, he had a hovering chair for crying out loud, he always always walks from point A to point B and so on and so on.
For somebody who is a popular character the last 30 years, one would expect to see him fly at least once in a while. But no, nothing.
So what he's an Eternal ? Who gives a hoot ? Not all Eternals can fly. For example : Thanos can not fly. He doesn't need to by the way, he has powers enough.