Unusual claims require extraordinary evidence.
As far as ancient alien theory is concerned, I'd contend there's far more evidence for that than the contrary. It also makes more sense in a lot of ways and answers to a few huge gaps in our historical model.
But hey... if a person isn't willing to explore the bodies of research on the subject to see what evidence exists, and it's not put in their face by 'educators', nor proposed in a convincing format on television where it's not subtly made out to be an entire field of presumption without solid facts to back those assertions, then why would anyone think there's anything to it right? In fact, one might suspect that it's quite possible that the manner in which the information is being delivered is being used to color things to the point that it mocks those who have put together any valid models, which just goes to suggest there could be political motive in making the whole field of study sound irrational. Or perhaps some corporate execs just feel it just makes the show more entertaining, and thus watchable by a larger audience, to mock its own premises perhaps.
Look, the ultimate point here is that all a conspiracy theorist is is someone who sees that there are problems with the given answers to things, potential motives to hide the truth, and some evidence to support a premise that answers to these inconsistencies. It's about reading the movements under the blanket of surface evidence and trying to determine what may lie beneath, with the knowledge that it is not likely to be nothing, given that there are movements there, and the movements are as of yet unexplained. My dog, in otherwords, is a great conspiracy theorist when he locates my feet under the blanket and nibbles on my toes.
More often than not, it's the best guess to the as of yet undetermined.
The body of evidence on UFO visitations is gigantic. And it leads to a lot of questions about agendas and who's piloting the craft. It very well could be us and if it is, boy there is there some amazing high tech stuff we have that nobody knows about and even if that's true, did we really invent it or just back-engineer it? If we know THEY are there due to the huge compilation of that body of evidence by those admittedly ordered to gather that evidence on behalf of the government, then why the secrecy? Why the failure to be on super high response against the potential for ET attack? Have we met with THEM already? Or are we perhaps already owned by them and always have been, albeit largely left to our own design and self-development? There's so much to this subject... I probably shouldn't have brought all this up again.
My point is, even if you feel that unusual conclusions should be backed by a huge body of evidence and that there isn't enough, I say perhaps it's more that you haven't been exposed to enough in some cases. Most of us are smart enough to utilize a natural sense of Occum's Razor in our logical processes and from there it's mostly a matter of how much we've been exposed to that determines where we fall in our belief. Those of us who explore the outer limits of possible explanations to things that nag us due to remaining unexplained, often do at least keep an open mind.
AKA, I'm a big enthusiast of Ancient Alien theory but no matter how much I feel it's the best model to answer to so many things we haven't been able to answer for about human history, I still do keep an open mind that it could all be explained in a more mundane manner. However, even an alternative theory we've been able to show is possible doesn't automatically mean that alternative theory must be true. It's not about just the small details but the big picture that emerges as well.