Except muting isn't 100% effective, because muting only works in-game. Sure, there's the Block option, but it's not 100% effective when the person can still be toxic to you in the post-game lobby, or send you a friend request to be toxic through PMs. (and, yes, there are some people who think that toxic folks could have a change of heart post-game. Dunno why.)
At any rate, your whole initial question is "why can't I be a jerk when there's an option to not see me be a jerk in chat?"
And the answer is; because you're not supposed to be a jerk. The existence of the /mute command is no excuse for being a jerk.