Why do you lose elo from AFK teammates?

I don't understand why a team should lose any elo when a random teammate (not in your party) leaves the game mid-match.

Shouldn't it be that you don't lose any elo, or that it is severely decreased?

If this gets abused, the elo punishment could be transferred in part to the afk player, instead of just being wiped entirely. aka -30 or -50 elo for leaving a game mid-match. Perhaps depending on how early in the game.

Nobody said there weren't any flaws in the system.

go create job application for riot