Why don't people believe in the Ancient Aliens theory?

Because it’s racist as hell.

If you ever read into the Ancient Alien theory or even watch the show a pattern starts to emerge, and it is this:

It’s never white people who got help from the aliens.

Because white people are just so awesome! We white people - and only we white people - are the ones who came up with science and engineering even though the same patterns of science and engineering were popping up all over the planet independent of white people, well…since white people weren’t present at any of these events telling these other cultures how to make things independent of white people science and engineering it stands to reason they had outside help - namely, aliens.

This is not only untrue, but also insultingly untrue.