While they of course aren’t perfect, IMO they are as a whole pretty amazing in their ability to measure intelligence/aptitude.
That has been proven to me over the many kids I’ve known over the years. Once I know something about them (their course loads, grades, parents, etc.) I can pretty much predict what they are going to get on the ACT/SAT, with surprising accuracy.
With kids of family friends who I’ve known for years, I can easily tell, well in advance, whether the kid will score in the 18-22 range, the 25 range, the 30 range or the 33+ range.
In fact, I could probably do the same thing just knowing the parents, and never having even met the kids.
Scores can be moved within a range (maximized or minimized), for sure. And anomalies or outliers will always exist. But the biggest predictor of standardized test scores is genetics.