Why do ranked games with afk players count?

I never play ranked, because I don't like actually loosing progress from something when I loose. But I really want to reach gold at least, because litereally everyone trash talks you when you are silver. So I decided to actually play ranked and reched promos for silver 3, I know it is bad, you don't need to tell me.

Well I played cho gath top and would say that I was able to keep the Nasus kind of small, making him go 0/3. Well on the other side of the map my adc, varus, just decided to completely feed the enemy Sivir and went afk after a few minutes. Leaving us in a 4v5. Well I certainly was dissapointed but not too much because I was sure that if someone in my team leaves the game wont count for sure. Well after the game I got surprised by the client telling me I lost the first game of the promo... with a game that was basically unwinnable. I can live with the enemy team getting LP for winning, since it would be completely unfair for them if they didn't get anything. But why the heck do unwinnable games count towards my rank.... what the hell.