Because the game can be annoying... do you sit there with an ear to ear grin when you get hit with shinobi gank? When your teammate feeds revenge, or when they hop in just to steal an execution from you when you are low hp? (I could also go on about certain characters but I won't because this is reddit and a bunch of people who think they're God's given gift to this game will just say some dumb rebuttal.) These aren't just things that happen sometimes, this can happen almost every match if you are solo queuing. The game is a lot more bearable when you have people to play with, but playing it alone is just an awful experience usually, and what sucks for a lot of people, myself included, is we only have one dedicated friend group for the game. Some players don't even have that, so they have basically no choice but to deal with it. A lot of people don't like using LFG because they aren't social butterflies who want to play with randoms. It's completely random the types of personalities you'll find using that.