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.
Smash Bros? Plenty of people play it as a competitive fighting game while others just like to goof around with their friends.
Funny enough, the two modes for Smash 4 were “for glory” and “for fun”