Why does an AFKer in a game leading to a defeat count as a loss in placements?

When a person goes AFK in your game, causing you to lose, why should that count as a loss in your placement matches? Since placement matches determine your rank for the beginning of the season, winning placement matches can place you higher and save you time, so why does a match with someone who goes AFK have to negatively affect my rank? When someone else goes afk, it limits our team's abilities and power, and we are more likely to lose the match. I'm just wondering why Riot does not implement a feature where they prevent the loss from counting in your placement matches if someone goes AFK, since it seems logical to me. Any explanation by anyone would be appreciated.