I believe the idea is that the entire curriculum is then centered around teaching the test, versus what kids actually need. As to whether that's the primary issue, no idea.
I'd be far more okay with standardized tests if the requirement also banned homework until at least Junior High. Otherwise it's a race to the bottom to see which schools can suck up as much of the children's and families home time to help improve the schools scores.
Some of the reason schools start homework so early is that parents demand it. I've seen parents complain that a first grade class isn't academic enough because the teacher didn't assign enough homework.
If you measure it less, you'll have a weaker argument for how the education system is doing!
Your comment is precisely the problem: objective metrics will always support their proponents' arguments for more measurement, while any upsides to a lessened objective grading will be difficult to show because they'll involve subjective assessment or less assessment.
Do not confuse the investigator's convenience in accessing a truth value for the truth value itself.
That argument has been made many times and in great detail. See for instance the work of Diane Ravitch, former US Assistant Secretary of Education (https://en.m.wikipedia.org/wiki/Diane_Ravitch)
School-funding and even whether a school closes or not is tied to these tests, so there would be some work involved in replacing that aspect of standardized testing in US schools as well.
