You forget that it's not 1v5, it's 5v5.
You forget that if I do carry, they will focus me.
You forget that not everyone is meant to play Jhin and go 31/2/15.
You forget that to carry your team, you got to be better than them by an order of magnitude.
You forget that the purpose of MMR is to balance you out with teammates equal to you - but if MMR is useless for a support or someone who isn't a carry, then what's the point?
By the way, I'm suddenly certain I'm somewhere from B1 to S5, solely because of how I can play a support. If I get stuck between those two divisions, at the very least I'm somewhere I belong, but, I'm sorry, the fact 17 (I've counted. That's how bad it is. I've had a lucky 3 on their team!) of my ranked games had an AFKer on my team is NOT my fault.
I'm aware we all have to improve - I'm not infallible, and I know that - but I'm also not someone who deserves B2-B3 because Jax decided he wanted to try, well, Jax out in a ranked game. Said Jax enjoys feeding the top-laner a lot.
Edit: Think of it like this;
You're 20% of your team.
To carry, you need to be 80%-100% of your team.
You need to be 4x-5x as good as your current MMR to carry - but what if you're only a rank or two up? After all, each league is only 25% better than the last, hence why even Challenger has issues climbing in Silver.
I don't mean to be vitriolic, it's just that my past few games and massive losing streak (I think I had like 7 wins in-between the entire 25 losses) have proven to me that, yes, it is a fallacy by math and logic alike to make some people very high up seem more important than they really are.
I'd also like you to explain now the "I must carry my match" mindset is good for the game itself? It encourages infighting, toxicity, blame, and worst of all... it encourages you to try and be better than the people you're carrying for sake of being better than them - not to improve, yourself.