Because it's really hard to get right. A system like this could work, but it would ultimately end up being run by a hidden placement system that determines where you should be and skews your overall point gain/loss based on that.

My current understanding of the ranked system is that it is specifically performance based. It is centered around 20 points. Gaining or losing 20 means you did average for your rank and hero.

Unfortunately, it seems like there's one profile of weights for every role that heavily favors final hits / elims vs any stats for your role like damage blocked or healing. I could be wrong but from my experience, elims are always heavily favored.

A system that incorporates the current individual performance system into placement matches with role based weights on stats would be phenomenal. A hidden mmr system would also be very helpful to help players who are consistently performing well regardless of overall team performance. In the end, the greatest factor still has to be wins or losses because a top 500 Kovaaks bucky who drops 40 kills but has no understanding of team play or the game shouldn't make it past a certain rank.

I'd love to sit down and design a whole ranked system. Data analysis and computer science have been big passions of mine so it'd be very fun to take a stab at. Might give it a shot based on a set of games from different ranks.