It’s not that it’s hard to learn, it’s that we don’t have a strong intuition. It’s not that difficult to know there are 12 inches in a foot and 3 feet in a yard or that metric is entirely based around factos of 10. But its the intuition of what an inch looks like, what a pound feels like to hold in you hand. Most people wouldn’t be able to pick up an object and say, “that weighs about a pound” or look at an object and say “that’s about 3 feet long” but a lot of people do have an intuition what an energy drink can looks and feels like and can imagine getting hit by one, a lot of people have picked up a pineapple at the grocery story, people have the intuition of how big a football field is or how big a city bus is.