Ghosts/spirits are surely the most "believable" thing there? They are, by their (super)nature immaterial, they are also localised and intermittent - it seems. They don't have to be the "souls" of the dead.
I suppose you could, hypothetically, explain contemporary alien visits in a similar way - the entities and their crafts are so advanced and/or different to anything we can understand that we are unlikely to obtain definite proof of their visits. Ancient "visitations" have the advantage of all of that, plus the fact that they happened thousands of years ago.
We would presumably have found definitive evidence of ancient, "futuristic" civilisations, not just "we don't know how they built "X", therefore aliens".
If people can move objects with their minds or predict the future, presumably we'd have proof by now.
The issue with bigfoot isn't that an unknown hominid or ape exists, simply that we haven't found definitive proof on a continent of 300+ million people.
I'm open minded but ghosts/spirits seem the most "likely" to "exist".