Standardised tests started as tools to do racism without directly doing racism.

Low test scores in the U.S. actually just measure the fact that it provides roughly the same education to every student instead of what most other countries do and filter students out if their scores aren't high enough in elementary school.

I would say standardized testing is a way for us to provide transparency about how we are failing our kids. The SAT and ACT stats you used are an example of that. This should motivate us to improve things, but like a lot of modern issues people just don't care enough to make it happen. Even so, being able to cite worsening outcomes supports people arguing for more investment in education.

I took standardized tests from elementary to college, and I remember their questions being objective, unambiguous, and relevant to learning topics much better than teachers' custom exams. I actually felt well prepared for the SAT/ACT/college thanks to the way they were used.

Teachers do need some discretion on what they teach, but without good standards you can easily have them just spreading their personal agendas. I don't want students learning about "the war of northern aggression", or that native Americans just chose to move to reservations actually, or that evolution is nonsense, or that abstinence is the only way to be safe regarding sex. Having expectations about what students should know at each grade gives a goal without stipulations on how it's achieved. Standardized tests then just measure it.