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.
Something that really doesn’t help is that although I am a gamer, I hear about how toxic many multiplayer environments are, so I never join them. I’ll only play with my real life friends, who aren’t going to call me a racial slur if I make a mistake or am honest-to-goodness bad at the game.
A toxic environment leads to droves of people who won’t tolerate it fleeing. Most people who won’t tolerate toxicity also would not perpetuate a toxic environment themselves—in other words, the toxicity drives out lots of non-toxic people. Now the proportion of toxic to non-toxic people is much more slanted towards the toxic.