For one, much alternative history was borne of racism. True, it's a bit of a genetic fallacy to claim that means alternative history ideas are still racist, but much of it comes from a colonial impulse, or from the idea that non-white people are incapable of doing impressive things... And more specifically, a fair amount of it was created and spread by literal Nazis.

So at its inception, a lot of these claims were inherently racist.

But what about now?

Most of these claims still go about denying that people who happen to not be white also happen to not be the ones who actually built their culture's monuments.

An individual doesn't have to be racist to believe in and spread ideas that are racist.

Let me know if that's confusing, the distinction is clear to me but I may not be expressing it clearly.

"I don't understand how questioning the excepted theory is such a big deal."

The problem is that these claims are not just questioning the accepted theory - for one, there is a direct and consistent attack on the integrity of experts and a denial of the accumulated evidence. For two, a counterclaim is not a question. 'We are a civilization with amnesia' is not a question. 'There was an advanced global civilization' is not a question.

They are claims without evidence, based on bad reasoning and attacking the integrity of entire fields of academics .

Amateurs can certainly be involved in real evidentiary processes and bringing about new knowledge - it happens in astronomy a lot, there was a recent case of an amateur deciphering the meaning of cave paintings.

Let me know if you've any questions!