Because it's virtually impossible to automatically detect int feeding.
Let's take an example from your post. Let's say we base it off of CS. If you have no CS by (x) time, and you're reported, you'll be punished for inting, because that's an easy way to check.
Problem number 1: Riot forgets to encode it to not happen for ARAM and random game modes that involve low CS. Now a fuckton of people got banned.
Problem number 2: Supports. Enchanters don't CS. Or at least shouldn't be.
Problem number 2.1: Riot encodes it so that enchanters aren't counted for this. Now inters are just feeding on enchanters. And they forget to code a new enchanter as an enchanter for these purposes, and half the community is banned.
There are just countless problems with an automated system for this.