They are compelled to think through what a good education is, and thus to take a more activist role in their children's upbringing. When parents hand the responsibility of educating their children over to the government, many lose some part of the incentive to maintain an intellectually vibrant family life, which is more important to a child's success than mere financing. Albert Einstein failed out of math. Mark Twain dropped out of school to work ships on the Mississippi. Not all students learn in the same ways, or at the same rates.
Some students excel in mathematics at an early age, others at a late age.
And some never enjoy math. Yet, somehow, the world continues to turn, and people from all learning groups continue to find personal happiness. Certainly I don't wish to argue that there's no way to measure academic progress. I even defend and tutor such tests as the SAT and ACT, though I note their severe limitations. The United States is a democratic country in which the majority rules by vote.