Can someone explain to me why someone afk'ing in the first 10 minutes affects my promotions?

So i have been playing league of legends since it launched but never had much interest in ranked play.. I have since changed that and this season I decided to do more ranked and see how much different it was. I have done ranked in the past.. Usually just the placement series and then i just kind of ignore it.

This season... During my placements series i had several bad games. i got 1 S game and 3 A/A+ games. with winning 50% of my games. 3 of those losses people left early or people just arguing with each other in your general toxic behavior.

Then i play 2 games and get 2 S games and qualify for promotions... i then get a zoe who leaves at like 9 minutes in because they died 2 times and just AFK.

She was reported... but now i have a loss on my promotion series because it was 4 v 5. Why does this count toward my promotion series? I cant control what other people do and i though the game was directly based on my performance. Shouldnt the game NOT count games in your promotion series when someone leaves? That seems like a really bad system to punish players for others leaving. These games shouldnt count toward promotions. And i understand that there would need to be a cutoff point because people would abuse the system if they were duo. Maybe duo groups get a penalty if one of them leaves early or something. But if this is how ranked is going to be I will just go back to draft mode because whats the point?