They already calculate when somebody dies, where they die, what they were doing, how much damage they do, how much damage they take, and it can be easily weighed against the teammates in the game. Nobody wants to lose LP, so purposely throwing a winnable game is plain stupid. And if they lose extra LP for their actions - as people who are underperforming heavily in ranks they're in should since they're not matching the skill level of their placement - nobody would want to throw terribly.
And it's not going to fix every single occurrence, but it will fix the most prevalent and obvious ones that need fixing. And why shouldn't it?
You say focus on things that can be changed, well this is a glaring problem with the game and ranking that can easily be at the very least patched up a bit to make it far less of a hassle. And it can be changed - what I cannot change is a 2/14/7 Yasuo telling me "fuck your mother" while he dives the enemy team again with only you and an AD Nid to back him up against 5 fed enemies. That's not a remediable thing, but there should at least be punishment for it and relief for those being affected by such a player.
One of the top reasons people leave this game is because there's no repercussions for doing stuff like this - even if you go 1/20/7, you're not going to get banned for intentionally feeding, and you can avoid any form of consequences by just not talking. At all. Riot doesn't give a shit about lack of communication reports and a 1/20/7 is a good enough score to them to make them look the other way and say that "They were at least trying", even if in reality they were repeatedly charging the enemy and would do one AA before you had to resort to trying to salvage the game by engaging in order to maximize the damage your team could ever do, so all those assists came from teammates trying to follow for the kill, and the one time they ran in on a low-health enemy.
Look, if there's no relief for these situations, there needs to at least be some actual consequences. The bollocks auto-punishment feature only checks chat logs, so these problems are going absolutely unpunished.