Schools are optimizing for standardized tests. There's no standardized tests on civics.
While people born in the US automatically have citizenship, my personal belief is that people should have to pass the citizenship tests to get the right to vote.
btw, I'm in Australia, and our voting is compulsory (or at least being enrolled and being crossed off as submitting a ballot paper, the vote itself is secret).
I think if we're going to set tests for citizenship for foreigners before they're allowed to become citizens, why shouldn't the same tests apply to locals?
I mean, you're not wrong, but that's not what the majority of schools' funding is staked upon.