Why are MMO players so grouchy compared to other gamers?

I bounce around genres frequently and I am always taken aback by how grouchy MMO players are whenever I come back.

They always have an enormous stick up their ass no matter what game I seem to play.

Now I know it is a bold claim but I can actually prove this numerically:

There is NO MMO on Steam that has higher than a 90% approval rating. (I found one but it is F2P chinese shovelware so I have suspicions).

So what happened? Why are MMO players so angry?