Ancient aliens, are properly classified as a pseudoarchaeology. We commonly hear these types of conspiracy theories noways in a variety of contexts. Aliens apparently arrived on Earth at some point in the past, leaving evidence of their visit behind because of how they shaped human cultures and the environment. They built the pyramids, they built Stonehenge, they left crop circles, and so on. Other apparent evidence is how different ancient religions have myths about alien-like creatures and gods. Also, there are artworks that depict nonhuman figures and architecture deemed too “futuristic” to have been created by humans at that point in history.
So why do these conspiracy theories exist? Well, the idea first teased our minds in the wake of science fiction stories popularized in the late 19th century. Desperate to make some semblance of these fantastical stories true, many people tried to gather evidence to prove their conspiracy theory to the world. One of the first of these people was Erich von Daniken and his book Chariots of the Gods? Unsolved Mysteries of the Past, published in 1968. His assertions that extraterrestrials built ancient structures for us was the first of its kind, and the theory quickly spread. Other, similar works would pop up for decades afterward, with similar arguments in favor of ancient aliens. One would think such musings, based on the naive want for science fiction to be fact, would be harmless fun. Unfortunately, the pursuit of evidence has led to several instances of vandalism and theft at sites that are believed to be made by these aliens.
It’s also rather concerning that the architecture that is being targeted as evidence comes from cultures in Egypt, Africa, and the Americas that have commonly been the targets of prejudice by other cultures. The assumption that these cultures weren’t “smart” enough to accomplish such technological feats can be rather insulting to its people. It also subconsciously reflects the same racist bigotry as colonists who treated those people as less intelligent and, as such, in need of their guidance to “civilize” them.