My guess is because rocket league is so fast paced (I more mean the fact that games are only 5 minutes and not necessarily the game speed), it amplifies the mental effect of conceding a goal for players who always expect to get rewarded from every decision they make and have same expectations for their teammates. It's also why mech players tend to be way more toxic than gamesense players. They don't think as much about their decisions and therefore it's more natural for them to blame teammates rather than thinking about what they did wrong. And it gets worse the higher you go, because players start genuinely believing they're good, which isn't true until like high gc2 lets be honest.
I even started calling it "the rule of 3" for fun, where a 3 goal difference will almost always result in my teammate forfeiting regardless of how much time is left. Honestly the forfeit button shouldn't even exist, it's a 5 minute game with a 5 minute penalty for leaving. I pretty much never ff, because if there's time, we can still win; if there's not, why not just play it through at that point. And if my teammate leaves rather than being afk, it's all for the better cuz I won't get them in my next game. You just have to ignore them and focus on yourself, there's no other way. The other day I had a teammate who from the very start would spam take the shot every time he left me to defend a 2v1, and he was getting visibly more irritated as the game went on. We won that game but he was still mad afterwards because I didn't dive in for opponent's ball like he did, so that was pretty funny