Because from the ground it doesn't look like anything at all. I've been there, and without knowing where I was, I never would have guessed that I was inside the structure.

Quoting from the wiki article

""The Richat Structure has been the subject of unsubstantiated fringe claims to be the site of Atlantis mentioned in the works of Plato.""

I wouldn't believe that claim either if there is no archeological evidence, but I'm wondering why there isn't actually some civilization that walked by, said "this shit is cool", and built a city there.

If I was an ancient pharoah I certainly would have ordered a city built there. Maybe with a nice pyramid in the middle instead of some burning man

Because barely one lived out there then and even fewer now.

Something with a name reminiscent of the Atlantic Ocean.

Maybe because it's in the middle of a desert.