You act like it doesn't go both ways.
Like when I play adc and go 7-0 bot Lane with 0 jungle help or mid Lane roams, and can't carry because either the other two lanes are both 0-15 or because every time I single handedly gain my team an advantage they throw it by trying to take an objective we don't have time to take. Oh we won a teamfight at our inhibitor turret, their jungler is up in 10 seconds, let's go Baron!
Or even worse, I score us an ace and we're right beside an inhibitor turret and none of maokai, Lee sin, or braum will step in to tank it without a minion wave when I'm there as an adc with IE, PD, LW, and boots ready to take it down (I've literally tanked and solo killed a turret with those 3 champions standing right outside the range waiting for a minion wave, meanwhile had they tanked it I'd have enough health left to stick around and take the actual inhibitor afterwards).
Just in general there are people who suck in any role. Also of particular note is all the times a tank yells at me for running from a 2v2 saying we would have won it when all his peel cooldowns are gone and they're both able to take me out. Sure we'd win the 2v2 in your fantasy world where they both target you (I see where you're coming from because when I run they do both target you and you see how long you live and think with my damage over that time we'd win), but in the real world they're going to kill me before I can get off 2, maybe 3, auto attacks and then kill you 2v1 after.