US customary is not "thousands" of years; temperature (Fahrenheit) only came out in 1724 for example, and many other units were only standardize much more recently:
* https://en.wikipedia.org/wiki/History_of_measurement#History...
* https://en.wikipedia.org/wiki/System_of_units_of_measurement...
Second, even with-in the system itself there was also sorts of inconsistencies. Until just a few years ago there were two "foot" units; one was recently retired:
* https://www.nist.gov/pml/us-surveyfoot
Here's another one: which weighs more, a pound of feathers or a pound of gold? A pound of feathers. Because feathers use one 'pound-system' [1] and is 453g, while gold uses another pound-system [2] and is 373g. But of course an "ounce" of gold (31g) is more than an "ounce" of feathers (28g), because Troy is 1/12 of pound but Avoirdupois is 1/16. In addition to the above "ounces" there is also "fluid ounces". And let's not forget about the third system [3], which got replaced by… the metric system in 1971.[4]
[1] https://en.wikipedia.org/wiki/Avoirdupois
[2] https://en.wikipedia.org/wiki/Troy_weight
[3] https://en.wikipedia.org/wiki/Apothecaries'_system
[4] https://en.wikipedia.org/wiki/United_States_Pharmacopeia
Also:
* https://en.wikipedia.org/wiki/Pound_(mass)
* https://en.wikipedia.org/wiki/Pound_(force)
> Think about it. Imagine you were never taught a measurement system. You find yourself needing to come up with a unit to make measurements for wooden planks you're going to use to build a house. Would you choose a unit as long as a meter? Or one as short as a centimeter or even a decimeter?
Millimeters. That single unit gets you down to very small scales (finer than 1/16") and to large lengths: four significant digits (9999mm) gets you to the equivalent of >32' (~11yd).
* https://web.archive.org/web/20200516223819/https://themetric...
As evidence for this Australia/NZ/EU do everything in millimetres in construction.