It is because it's still based on ERA which doesn't take batted-ball luck, defense, etc. into account. And you're the person that thinks xFIP and SIERA are exclusively predictive stats and say nothing about the actual quality of a pitcher.
https://medium.com/ro-baseball/earned-run-average-is-a-bad-statistic-and-should-be-retired-97e4c9feaf5f#:~:text=There's only one problem%3A there,completely out of their control.