But you do.. Sort of. One of the original signatories of the treaty of the metre, and all of your silly units for length and mass at least are defined in terms of SI units as of 1959 [1] [2].

So when you give someone an inch, you're specifically giving 25.4mm ;)

[1] https://www.nist.gov/pml/weights-and-measures/si-units-lengt...

[2] https://www.nist.gov/sites/default/files/documents/pml/wmd/m...