Community: "Why is there no early surrender option?" / Riot: "Because it's exploitable."
This is basically the gist of it: if there were early surrenders available due to AFKers, premades would exploit it to waste peoples' time.
Additionally, there's a reason why you don't get reduced LP loss when you have an AFKer: EXPLOITERS! If one team is losing, they'll just tell one or two players to AFK to avoid LP decay.
Riot, please put something like this up as a sticky.