Genuine Question for any Riot Devs who might see this: Why LP loss in Leaver games?

I was playing a ranked game just now and we had a Lucian leave at around 11 minutes because of a single bad fight mid. The funny thing is our team rallied really well and if we had been still 5v5 we actually could have won the game easily. But because he left we lost and everyone lost LP.

The Lucian was gone for long enough that his leaving had been recorded and his named redmarked, so the system obviously knew we were handicapped pretty badly in that game. It's one thing if your jungle or top laner leaves, but losing your ADC is basically a death knell. Now, having played DotA 2 for a good while I know it's possible to invalidate games that are counted towards MMR so it's not like it would be impossible to register a game as invalid for LP loss/gain purposes.

I know what some of the reasoning might be against invalidating: "But what if you win 4v5?" to which I would say: the sheer number of ranked games I've won is vastly outnumbered by the ones I've lost when the situation is 4v5. I've been playing since S1 so I'm not lacking in sample size. It's not impossible to win obvious, and in Blind Pick it's fine. Ranked, though, is a different beast and with a leaver you end up with 4 people being punished with LP loss for one person's selfishness and that seems incredibly unfair. I'm not really a hardcore ranked player, I love playing league but Ranked always got a little too toxic for me to enjoy as much League deserves to be enjoyed.

That being said I'm genuinely curious as to why games with registered leavers still deduct LP from a players rankings even though they're clearly stuck in an unfair game. If a Riot dev could answer that question I would be very appreciative. I'm sure there's some kind of reasoning behind it, but on the off chance there's not I really feel like League of Legends could benefit a lot from a little more attention to fair play in Ranked.

EDIT: After reading through a lot of this particular section of the board I see a large number of players complaining about this same thing, getting LP loss even in leaver games. This really seems like something the community wants to be changed. I know Riot employees like Rhojin and Tantrum read through these, I would really appreciate some kind of answer.