I wonder if it is an internal/external optimization. The whole point of metric is that it is built around the number system, leading to easy conversion between units.

The whole point of the American unit system is that they are optimized for specific use, but definitely not for conversion.

ISO paper sizes are a great counterexample, actually, because they took an application, rather than a conversion-centric approach. I.e when you print a signature you do so on a larger piece of paper, and then you fold, crop, and bind. Each fold cuts the paper size in half.

So ISO paper sizes are much more like American units than American paper sizes are.