Used a couple of US recipes recently and most of the ingredients are in cups, or spoons, not by weight. This is a nightmare to convert. Do Americans not own scales or something? What's the reason for measuring everything by volume?
Cups are great, the real annoyances are pints, quarts and gallons. Really no reason to use any of those when you can just state it in cups. Then we threw in fluid ounces for that extra layer of confusion and conversion.
I think that is the historical reason for cups. I saw another commenter post that a measuring cup is approximately the volume of a teacup, and those do tend to be about the same size.