Because it doesn’t seem to matter currently if you play ranked games or casual games, the general experience tends to be the same. But one has numbers and things to go with it. You still get people playing to win in casual games and you get people dicking around having fun in ranked games, and the ranks don’t necessarily indicate how they play as a team and a whole bunch of other things that make it less than ideal.
And then you get the tryhards shitting up the for fun list, unless you do something that makes it explicitly for fun like smash having (more or less) tourney rules in for glory and having items on and whatnot in its fun mode.