The problem is MOBA matches last way too long. If you feel like you're stuck on a bad team in something like Overwatch, it's only for 5-10 minutes tops, but MOBA matches can go on for over 20 minutes, and that's a lot of time to feel like you wasted because you feel your teammates aren't pulling their weight.
Not saying I think it's right how toxic those communities are, just saying I can follow where that toxicity stems from.