Spoken like someone who’s never played the game. Get ready to be banned for no fault of your own, just because your three teammates decided to report you for the lulz. There are people who post on /r/dota2 complaining that they get banned from the game for nothing but picking techies each game. They don’t even say anything. People just report them immediately.

By the way, streamers are one of the most high impact users of the game. They are also one of the most unfairly targeted by report systems. This problem is extensively studied and very tricky.

Riot does exactly that.

Okay something tells me you've never played an online game, like ever?

The chat is not the only way you can troll your team mates and cause them to rage quit (or scream at you and get banned themselves). I would actually say that the griefing created by people using slurs is actually pretty small.

You can be silent and ruin hundreds of games. Distinguishing that from genuinely bad players is not really easy.

YouTube, Facebook and others have several orders of magnitude more money to throw at the problem than an indie game company.

That’s why Riot’s level of progress over the problem is so remarkable.