Because Riot, the boards, and Mods only interested in dealing with BLATANT verbal toxicity. I've said for 2+ seasons now that they need to deal with griefing players, trolls, intentional feeders (hard and soft), and less blatant verbal toxicity. All I get is hate, bs excuses, and toxicity. Until the boards, players, and Mods wake up and realize how toxic it is to NOT address this stuff and force Riot to react, we'll always have sad true stories like what you are describing.
I've made over a dozen posts about this issue, cited plenty of actual gameplay examples, and explained how it's not actually hard to tell if someone really is either hella boosted or inting vs if they're having a bad game. But sadly everyone seems like they'd rather keep protecting griefers, trolls, and hella boosted players than fix the problem. Because somehow even though you very clearly explain the differnce... you might ban, demote, or otherwise punish the adc that goes 1/6/0 in lane while being 4 man camped with no help from team in addition to going after the toplaner that 1v2 keeps running it down lane without trying to win the fight but currently gets excused because omg he auto'd the enemy once so we can't know he's inting. Uhm... you can tell the difference very easily - and I've explained that many times. Still people don't listen. They just see someone mention inting or punish and bam. You're wrong lalala, can't hear what you're saying, you're still wrong, I don't care how logical and valid your argument, lalala can't hear you. So freaking dumb and annoying of them.