To some degree, this is a storytelling conceit.
The Marvel Universe is meant to be the real world, our world, and to reflect it as much as possible.
In our world, whether aliens exist or not, they are generally not believed in.
Once you get to the point where everybody in the Marvel Universe believes in things like aliens, the whole landscape of the world begins to change in ways that make it into something very different from our world.
We don’t really want that to happen.
And given that there are moon landing deniers and such in the real world, this isn’t too much of a stretch.
So super hero robots and people who can bench press mac trucks are ok for the common person to believe in but aliens are...