That's in a fraction of games. Most times you end up getting someone in an off-role or somebody clearly way above their League. In fact, that fraction that's happening? That's them. THAT player is "evening out" based on their true MMR. And YOU get to suffer for it.
Me getting a 90% winrate (recently) gold 4 in a Plat 1 game who's only 90% because, oh I dunno, he's been playing against fucking silvers and golds? That's not me getting "even'd out", that's me getting fucked.
That support main autofilled into jungle who doesn't say a word? That's getting fucked. The D4 boosting his G4 buddy whose head was recently caved in? That's getting fucked. That unranked that was performing pretty well in completely unstructured games? That's getting fucked.
That excuse you're making is utter bullshit and you know it deep down.