> Every American kitchen has measuring cups, few have scales.

There's no reason Americans can't buy scales.

I got mine for $15 at Costco 10+ years ago. You can still get them from Amazon or Walmart for less than that.

> It may also be that US measuring cups are "more convenient" sizes than the equivalent metric ones would be.

When measuring out flour, I don't use a measuring cup at all. I put a bowl on my scale, hit the Tare button to zero it out, then add whatever number of grams of flour I need to the bowl.

> For flour, they often specify "sifted" which removes some of the variability.

That's gotta be awkward to sift into a measuring cup.