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 uses/used “For Fun” and “For Glory”, which I thought was pretty cute.
I think there is an issue with saying that ranked is “playing to win” though, since people in non-ranked games are still trying to win. They probably don’t want the pressure of ranked, or maybe just don’t want to play the meta.
And also people in ranked may still be having fun