Because this is basically what happens:
Let's say I pick a non-meta champion, like Evelynn mid. A teammate that is normally not toxic starts harassing me for the entire game, and/or possibly even rage quits. Let's call them Bob. But Bob doesn't use any racial slurs or anything that would trigger a punishment quickly (within 1-2 games).
So I report Bob. Bob doesn't have a history of being toxic, so Bob doesn't get punished. Bob doesn't see another Eve mid for 100+ games, so he's only toxic in less than 1% of his games. My report weight goes down because Bob doesn't get punished, making my reports more meaningless.
And then in my next game I get assigned "mid" and pick Eve again, and this time I meet "Jill," who is exactly the same as Bob, and the cycle continues.
Riot says "toxic players turn people toxic and see more toxic players in their games" but... There are also not-toxic things that can turn people toxic. Picking Eve mid is not against the rules. But it makes a LOT of players harass me for it. And none of them are ever punished, because they are not normally toxic. And in those games where my team DOESN'T tilt and harass me? I actually do good. The pick is fine. But in most of my games I'm basically getting the same gameplay experience as one of those toxic assholes who turns his entire team against him by being an asshole in chat. Except I'm not an asshole in chat, and I'm not breaking any rules, so I'm never going to be punished and removed from the game. But then, neither are any of my toxic teammates, because they're "just having a bad day" and don't "normally" act that way.
At the very least, the system should be able to detect things that bring out toxicity which **_are not against the rules_** (eg: non-meta summoner spells, builds, or champions) and then punish people severely for harassing these people, since it's going to happen to them almost every game otherwise, and as such they're going to have a skewed view of the community (they will think this community is way more toxic than it actually is, and probably quit because of it).
(Getting sick of people harassing me and even sometimes RAGE QUITTING and getting away with it with no punishment)