Is it the matchmaking or because there's so many people playing the game ?

I'm TIRED of games where you try to win and patch up your teams failings and feed and you just know you're not coming back from a 0/10 mid or 0/15 bot. When I jungle I feel like I'm trying to patch up 100 bleeds and you can't do it all by yourself. I gank top bot dies, I gank bot mid dies, etc. There's always that one feeder or the triple happy meal where al llanes lose. Yeah, I know, don't gank the feeder, try to get other lanes fed, but when the enemy is a 10/0 leblanc that RQ your adc and he's done for (god forbit adc's get mr or something), there's not a lot you can do and you're also playing 4v5 with a champion like minion on your side. I'm in plat 1 right now, didn't play ranked for 6 months because of garbage q, but wanted to get my diamond border, because I have it from last season too. Games here are just so frustrating and you feel like life is being drained from you when the lanes you helped still manage to lose or when you have something like a 2/18 vayne. It's just so tiresome. Honestly, I'm starting to see that when I lose or win couple games, I legit played the same, the only reason I won or lost some games is that there wasn't a 0/10 idiot on my team - maybe that means I should work on myself and not tilt so fast too. Also when you check feeders match history, they play a champ they didn't play in last 20 games, which I won't ever in my life understand, because not only they ruin games for 4 other people, they also lose LP. Idk if it's the matchmaking that's forcing you to have 50% win rate or is it just the huge size of players so it's bound to have bad ones too. Though I see some accounts having 80-90% win rate and that is just fascinating to me, yeah they're probably veteran players, but still.