Why do they let people troll so hard?

I don't play ranked because I don't want to care when people on my team troll. Which happens 1 out of 3 or 4 games. Way too much even in normals where it doesn't matter. But, i decided to play the 10 games to see what happens and in no less then 3 of the games people intentionally quit, jerked around while not attempting to win by hiding in bushes, or literally just fed during their games. Why? Why aren't these peoples IP addresses perma banned from the game? WHY? This makes no sense. Allowing people to behave this way is retarded. It's so easy to combat. Ban anyone in ranked who behaves this way for a few months, and forever on any second offenses. Id bet any amount of money the instances of trolling would drastically go down after this made the moba news arena. And streamers started getting banned for their trolling and so forth. They intentionally allow this. I don't get why they encourage this type of behavior. Reward it even with smurf accounts. I will only play ranked now since it doesn't matter after playing a series of unwinnable games because people on my own team choose to lose.

If anyone is wondering I only play because I'm saving up for a serious gaming pc and all i have is my laptop. So i can't play any real games that require graphics.