In my case, it's age. I only have so much free time during a day, and ruining it on a shitty Dota2 pub match just makes me feel like trash for weeks later. Even today, I cringe when I think back on some of the stupid things I did in that game.
The last thing I need is more embarrassing memories running through my brain when I'm trying to sleep at night.
Damn! Matchmaking is definitely really hard at high MMR levels, and I remember back when I read reddit about how many people would post angrily about matchmaking - there's no easy solution to that problem at that point. I remember watching a twitch stream after TI9 where Ana was fountain farming an opposing team until they got an abandon. (The deserved it for being asses in the allchat to be honest, but the skill gap was still obscene.)
But at the same time, your experience is exactly how I figured things would go if I ever tried to play Dota seriously. When I played football in my teenage years, I realized that eventually, you reach a point where getting better stops being fun because everyone else is equally talented and trying their hardest to get to the next level too. At that point, I realized that I just wanted to have fun and stopped competing. Still played recreationally in my regional league though. We had terrible results, but losing on a bad team was a lot more fun than winning on a good one.
Glad to hear that Dota helped you kick all your bad habits. It's always good to find a silver lining, no matter how crappy the situation is.