people in teh USA must despair of us though, with our mixture of miles and metrics when it suits us
It confused the hell out of the ESOL students (I was teaching IT to them, I am not an ESOL teacher) when I gave them a shopping list to compare prices at different supermarkets.
Not one could find '1 pint of milk', 568ml yes, a pint no. Did anyone else not notice when a lot of shops stopped putting 'pint' on the packaging?
As for cups, I do have a set of measuring cups, but as long as the entire recipe is measured in cups it doesn't matter what size the recipe will still work.
It's like the basic sponge mixture being 4oz of flour, sugar and butter with 2 eggs. To get a perfect sponge you should use a balance scale and ensure the dry ingredients are the same weight as the eggs, but 9 times out of 10 it doesn't matter.
I think, but stand to be corrected, that some US conventions are due to the history of immigration, migration, settlers etc.
Weighing scales would be heavy and expensive, if you are using a balance type you need to carry a set of weights. If you are traveling hundreds of miles in a covered wagon it's just not practical, on the other hand everyone will have a cup/mug/beaker.
Ditto eating with just a fork or your hands. You can cut up everyone's food with a single knife before you eat, put it down and let everyone use a fork or their fingers or a spoon. A wooden spoon is light, not dangerous for a child to use and can be replaced from a tree you pass, a knife isn't.