What does a splitpusher have to show for it?
Maybe they had a couple deaths, not many assists or kills, but they took down 6 turrets. Their rating would be awful, but they focused on objectives.
If rating analyzed objectives and actual gameplay, I'd say why not?
But it analyzes numbers.
If you're Shaco (or any other number of champions) who can keep players chasing you while your team is busy destroying their base, you won't get rewarded for that, even though it was your play that allowed your team to do what they needed to do.
Too many people would be unrewarded for things that they do for the team.
*I'm a support player who almost always has a great rating with either a win/loss, but I don't believe I should be rewarded every time I lose.