See all the reason JoOo listed below.
This game is free-2-play. It takes about 2 minutes to create an account. The character length for a user name is very long and includes lower, upper, and numeric so the possible combination of username space is in the billions if not more. Plus Rito does purge old inactive accounts and release their usernames (except perma-banned ones)......every once in a blue moon.
I'm not sure what does pre-made has to do with account sharing policy. It is against the rules whether you feed or not. And on that note, just because your team is okay with someone feeding because he is playing someone else's account that is well above his level (whether a completely new player or someone not at that MMR, above or below) does not make it okay for the enemies. They queued up to play with people their MMR, not a feeding newbie not playing his own account.
Besides, it will open up a slew of "I'm not responsible for this toxicity. My friend was playing my account" support ticket Rito shouldn't need to spend time dealing with.