Exploitation is why Riot is hesitant to do these systems.
About to lose a game? Someone who doesn't care just leaves, now everyone loses less. Right before you win a 5v5? Someone quickly D/C's (which I think will get you a red name) and your friend gets extra LP. Yeah, there could be some systems in place for that too, but then you're making something, that requires you to make something else, which may require you to make even more stuff, and that costs lots of blingage.
I don't agree with the current system either, as we see plenty of perfectly good matches spoiled due to these things. Also, it would be hard for those things like "My house is literally on fire" "My kid is choking on something". Those life emergencies are gonna come up, and even harsher punishments for AFKs would be...odd to deal with