The problem that keeps blizzard from acting on throwers (and therefore the problems that stem from throwing) is that it is near impossible to tell the difference from an intentional and unintentional thrower. They can tell when someone is way underperforming, on their own, no reports needed. But the last thing someone on a huge tilting lose streak needs is to be auto banned or even just auto kicked way further down in Sr for “throwing”. Sure, they were underperforming to the point of causing their teams to lose, but the insult to injury would be crazy.
Are they simply playing while tired or intentionally throwing? Are they playing a hero they might think they are better with than they actually are or intentionally throwing? It’s impossible to tell.
So they will act on the completely obvious cases, with help from reports to varify. But they can’t act on anything too grey which still allows those players to either knowingly or unknowingly ruin games.