Single-shot standardized exams are well documented at not working. It's a top-down call for a commoditised body of knowledge expressed through a skill that can and is gamed. It's illusory because you could get approximately the same results with a random-number memorization contest or board game. It's an artificial bottleneck designed to filter, almost nothing more. (In fact, by forcing this commoditization you starve the labour force of chance at meaningful early differentiation.)

You tend to find these tests deployed and celebrated in systems with a decidedly non-meritocratic political core, in large part to distract from the fact that the rulers' kids' scores don't matter.

I say this, by the way, as someone who excels at tests.