First, lets clear up a misconception you seem to have.
Actions are actually punished more harshly than words. Gameplay toxicity starts at a 14 day ban for 1st offense.
Verbal toxicity starts at a 10 game chat restriction, exception of zero tolerance.
Gameplay though takes longer to punish because Riot doesn't want to punish legitimate bad players. So determining intent is important.
As to why mute isn't enough. Well, there is a simple saying. "I mute so I don't have to deal with you. I report so no one else will have to." Also, unless you are preemptively muting everyone; potential hurtful or demoralizing things could be said and seen.