For the most part?
People figured out it wasn’t worth socializing with other players.
… not aided by many negative experiences over the years.
So most avoid content that requires socialization, because of the uncomfortably high possibility (exact numbers & probabilities don’t matter, the perception of that possibility does) that they could run into a jerk who will spoil their whole day.
Why play with others and risk having a bad time?
Play by yourself, then you can do whatever you want.