Because then education ends up being about the damn test instead of developing beings. The goal shouldn't be passing the test, it should be learning how to learn, plus a good base in general knowledge so that you can apply critical thinking and successfully integrate new knowledge and experiences.

It doesn’t promote knowledge, it’s gamed, and it’s used to further discriminate against schools that need funds.

It's been successfully demonized by teachers unions over the years. While there are limitations, with the medium, it's far from useless. It can't fully assess mastery of a subject, but it can determine basic and applied understanding of one. It's a very simple step to expand from student competency to teacher competency.