Why do I get a loss when someone afks?

Just had a Corki afk the entire game. Gone from the beginning, right up until the end. We tried to remake at the start but it wouldn't let us even though Corki was not there. He reconnected, then disconnected right after within the first 3 minutes, never bought an item, never moved. I'm honestly just curious as to why this counts as a loss and I have to lose LP for it? Not gonna lie, I'm a little salty and just was wondering if anyone could break this down for me.