If they did, they would perma ban a lot more people.
Some offenses warrant it, with zero tolerance.
But if they cracked down too hard, they would have less players.
There is no reason they shouldn't other than that, or laziness or incompetence.
If the support wanders into my lane at top at 5 minutes, that players account should be done.
If someoen throws a fit because they dont like how someone is playing and says "I'm done" and sits at base, that players account, ALL OF THEM, should be done.
I dont see why my time should be wasted because Riot doesn't want to clearly explain the rules of conduct to all new players from here on out, and come down hard for people who want to bail on a match.