Because their system is mediocre.
I more-or-less quit playing because of this reason in particular. Got into a game a few months ago, went to my jungle and pre-made not refused to leash. I literally said nothing about it and they randomly start attacking with "you fING stupid dke leash yourself" to which his buddy replied "yep were not helping for fggt with s*"
They lost bot, hard, and every mistake leading up to it they randomly blamed me saying things, in all chat, like "we're doing bad because of this dirty fING fg on our team, can't do sht probably because she's mentally fked, she has to be if she doesn't like guys"
Despite me being 2/0/4 and assisting other lanes just fine. Enemy team apologized and said they'd be reporting the two as well.
Out of curiosity about a month later I looked them up. They had been playing non stop since then, which mean if they did receive any punishment, which I doubt, it was nothing more than a chat restriction.
I'd rather not place any more time or money in a game who's company has a punishment system that doesn't punish fair and evenly, and seemingly works based off nitpicking.
People can be mildly annoying and toxic (as shown by reform cards) and being bumped to 14 day/perm bans. Then we have people like the aforementioned who may or may not receive a necessary punishment, despite nonstop harassment of individuals.