That's because the punishment is so minimal that people do not care. I had a jungler this season get mad and leave the game. I reported him and moved on to the next game after we finished which was like 5 minutes later hoping he would come back. He was in my next game too. He did the same thing too played 15-20 min and then just left. Never said anything in game and we were winning both games too.
Make the penalties like this and you will see an immediate change in behavior
Leave a ranked game early 1st offence - 1 hour ban
Leave a ranked game early 2nd offence - 24 hour ban (if within 30 days of prior ban)
Leave a ranked game early 3rd offence - 72 hour ban (if within 30 days of prior ban)
Leave a ranked game early 4th offence - 1 week ban (if within 30 days of prior ban)
Leave a ranked game early 5th offence and all subsequent - 2 week ban (if within 30 days of prior ban)
Dodge Que 1st offence - 30 min ban
Dodge Que 2nd offence - 2 hour ban (if within 7 days of prior ban)
Dodge Que 3rd offence - 24 hour ban (if within 7 days of prior ban)
Dodge Que 4th offence - 72 hour ban (if within 7 days of prior ban)
Dodge Que 5th offence and all subsequent - 1 week ban (if within 7 days of prior ban)
Also make the leaver timer longer so if your internet drops or your power goes out for a second you have more time to reconnect without penalty. 5 min should be increased to like 7-10 because if you need to reset your router that takes like 2-5 minutes alone.
The current penalties are 6 min(1st), 15 min(2-5th), and 20 minutes (6+) that is nothing. That is the problem with the penalties who cares you get a 20 min ban that's not even the length of the game.