Partially because Universities insist on making professors both teach and perform research (for the most part a few do have a real distinction between teaching and research but most still require at least a token class from most of their researchers) which isn't what most people go into a PhD program to do.
That's not universal, it factors in at some points at least at some universities, my wife is going through her reappointment after her first year as a professor at an R2 HBCU and teacher evals are part of it from what I've heard of the process there and she was definitely hired as principally a research oriented professor.
And, let me add, “research is (often) broken”:
https://m.youtube.com/watch?v=9yPy3DeMUyI&t=913s&pp=ygUSUmVz...
> Partially because Universities insist on making professors both teach and perform research
That alone would not be problematic. The real problem is that they insist on it, but only evaluate them on their research. That doesn’t create an incentive to spend time on getting better at teaching.