I will not deny that evidence, hard physical evidence, might exist. It's just that I, personally, haven't seen it.

With 75% of this planet covered by water the evidence could well be at the bottom of an ocean. If it hit a mountainside it could be buried under tons of rock. Wreckage that is seen might not be recognized for what it is -- can you tell the difference between the wreckage of a twisted, burned out 1930s well-drilling rig and a wrecked jet engine? At first glance? From several hundred feet in the air or going, perhaps, 50 mph along the ground?

Government coverups aren't needed if the evidence isn't accessible or recognized in the first place.