The short answer is that there are historians and historian-types that specialize in pseudohistorical material - although generally in tracking the history and historiography of the development of the pseudohistorical ideas and the debunking of them. The main name for that kind of thing would be Jason Colavito, who wrote The Cult of Alien Gods: H.P. Lovecraft and Extraterrestrial Pop Culture and numerous articles. Other historians of note are Jerome Clark, author of The UFO Encyclopedia: The Phenomenon From The Beginning and Curtis Peebles, author of Watch the Skies! A Chronicle of the Flying Saucer Myth.

While you do get occasional works from accredited and rational inquirers that become convinced of Erich von Däniken's "ancient astronauts" theory - key example there would be Josef F. Blumrich and The Spaceships of Ezekiel - it is less common to see organized academic scholarship around this kind of pseudohistory than among some other fringe theories. I don't really want to delve into speculation as to why, although it's worth pointing out that many of the other examples of fringe or pseudohistorical beliefs tend to have strong political, religious, and ethnic identity components. That is less common in "ancient astronaut" material, although not unknown since you had groups like the Fourth International Posadists with texts like Flying saucers, the process of matter and energy, science, the revolutionary and working-class struggle and the socialist future of mankind, and in particular to highlight one segment of your question:

The ancient aliens thesis is a belief system with a lot of really committed people behind it. It includes thousands of people in its official organization, and millions of people who consume its "scholarship". In some quarters, it verges on a religion.

In some quarters it is a religion, at least in the case of the Church of Scientology, Raëlism, and the like, and those organizations are invariably very young as far as religious sects go - and this tends to run into one of the major issues with "Ancient Aliens," the lack of robust evidence.

History as a discipline relies very strongly on literary evidence (published books, manuscripts, journals, diaries, letters, monumental inscriptions, etc.), and UFO religions appear only after the extraterrestrial zeitgeist really takes hold after the turn of the century - before that, you get similar groups, like Ohaspe and Theosophy, which prefigured (and inspired) a number of later developments with regards to ancient aliens contacting Earth, but lack a lot of the specific expressions and language we've come to associate with them - they just didn't have the same idea of "extraterrestrials" without 50-60 years of pop culture gestation. Lack of written evidence of alien contact is generally a major damper on research - especially the farther back you look, since it generally requires specific academic expertise to correctly interpret, say, Medieval French manuscripts with strange objects drawn in the margin, or ancient Egyptian murals. This is less of an issue for some of the other beliefs you've cited, since 1) there is a robust historical religious literature for subjects like Young Earth creationism and Jesus mythicism, and 2) you can't have Nazi revisionism pre-Nazis (in most cases), so no-one goes looking for documents related to ancient Nazis (generally speaking).

The other aspect of "Ancient Aliens" effort generally focuses on complementary or supportive evidence - that is, out of place artifacts from archaeological digs, gaps in the historical or archaeological record, euhemeristic reinterpretations of mythology, and especially personal accounts from people that claim to have been abducted, etc. At best, eyewitness accounts and archaeological evidence can be difficult to use in legitimate historical research, and are often adjuncts to the written record, but again you run into the same sort of problem when looking for historical records related to ancient aliens: you're looking for a needle in the haystack of history, and that's assuming what you're looking for is there are all. That's part of the reason that ufologists like Erich von Däniken tend to cherry-pick their evidence, and helps foster conspiracy theory adjuncts to the basic "ancient astronauts" hypothesis (i.e. the evidence should be there, and if the evidence isn't there, then someone is hiding it - and that's basically the premise for The X-Files).

Which again, doesn't necessarily mean there aren't serious people with PHDs working on ancient astronauts, but there is rather less raw material to grapple with, and the field of history grows very broad requiring more specialized expertise the further out you go looking for evidence, and the scope of that kind of research can be very limiting.