I think humans are a very curious species of great ape in that -- even when grown up -- we love to play "let's pretend", and we even so very often convince ourselves that our games of let's pretend are not games, but are actual truths.
Let's pretend the moon landings were hoaxes. Let's pretend jet exhaust are chemtrails. Let's pretend aliens built the pyramids. If you're not so accustomed to the phenomenon of let's pretend that you are unable to look at it with fresh eyes, it is astonishing how self-deluding our species can be -- and even more astonishing how tenaciously we can cling to our delusions.
The difference between a child and an adult is that a child often knows his or her self-delusions are just a game while an adult has mastered the art of let's pretend to such an extent he or she cannot be shaken out of it.
Were as many people just as delusional about mundane, practical matters as they are about moon landing hoaxes and pyramid builders, it would be routine to walk down the street and see people trying to mow their lawns with rose bushes, or water their children to make them grow. Humans are the least sane species of the great apes.
I sometimes think a cause of our seemingly endless games of let's pretend is that so many of us have such dull and boring lives that we desperately try to escape from them into dramatic fantasies. Better to believe in chemtrails than face the fact that somewhere deep down you suspect you don't have a life worth living.
Most likely there are other reasons for so many games of let's pretend, too. But I think escapism is one of the major motivators for the games.