^Huh? I did the exact same thing you were doing: mentioning something that is a property of fictional worlds. I wasn't talking about whether it was believable, I was just saying that it's something that many works of fiction assert to be the case.
For example,
War of the Worlds: The Series asserted that humanity had forgotten the global alien invasion and devastation of 1953, partly because of some kind of alien-induced amnesia, but largely just out of denial and an unwillingness to believe in aliens.
Buffy the Vampire Slayer asserted that the citizens of Sunnydale chose to deny all the supernatural things happening around them because they just didn't want to think about it.
Doctor Who in the modern era has routinely had humanity forgetting alien invasions or dismissing them as hoaxes, from the first season -- when the Slitheen invasion was promptly written off as a hoax -- to the most recent season -- where the Doctor said that the trees had overgrown the planet to save it from disaster multiple times, but had been forgotten every time. And many, many times in between. It's a common trope in mass-media sci-fi that people want to live in comfortable ignorance. And of course
there's a TV Tropes page on it.