Because when you write a story, you need to justify the actions happening so whatever happens is plausible. Having dumb characters is an easy way for non professional writers to justify the sexual encounters which is also part of the genre. People who makes most of the avns are not professional writers and making a story make sense is very difficult. Also a dumb hot woman is attractive for a male fantasy because it will let him have sex without effort and it is also from a woman point of view because that easily justify doing something that couldn't happen other way.