I think for a lot of these things, it's legitimately hard to imagine the implications of not knowing something that you've always known. Fractions are a perfect example. I've seen multiple adults in our age range that have absolutely zero concept of fractions. You throw a / in a printed number and their brain goes blank. They might generally understand what half means, but "which is bigger, a third or a fifth?"... deer in the fucking headlights. Most people probably never really need to do actual operations on fractions, but just try to imagine not having an intuitive understanding of ratio and proportion. The technology we use has certainly advanced, but the raw capability of the typical individual? Scary thoughts...