This is because league of legends has only a small percentage of truly toxic players according to riot. So what need do they have of a system to compensate players for lp loss from afkers, and legitimate trolls like the one you had? (Definitely no sarcasm in that statement...) I mean clearly such a system would only be abused by people who want to get out of losing lp for clearly bad matches. It's not like they can make the ban / restriction system more sensitive to give incentive to not abuse it. (Alright the sarcasm is out of my system now)
Intentionally feeding and replacing all your items with nashors tooth on Sivir is a severe case of trolling. Honestly it's worse than flaming or negative attitude (Nearly everyone does this often be it actual ingame chat or irl where they can't get banned for it. The game's very nature induces it. League mixes players who are just in it for "fun" and are not too concerned about winning or bettering their gameplay with players who define "fun" as winning. That's the perfect formula to generate rage. Not to mention how prone games currently are to outright stomps.)
The ranked system needs to have some sort of filter to keep non competitive players out of it. Which speaking from the perspective of a programmer is not a easy system to design or implement. If it's too strict no one can play ranked and the whole community rages. If it's too lenient the system is a worthless waste of budget. There is also the matter of what criteria must the player meet to be judged as "competitive". A certain kda? Perhaps apply a formula that calculates some set magic fitness value out of gold gain, damage done, objectives taken, and other things? I"m sure riot has enough data to analyze and make such a system. The question is would it be worth the cost to do so from riot's perspective.