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 ;)
And yeah. I mean 10 is only divisible by 2 and 5 where 12 is much more flexible in giving us 2, 3, 4, and 6. At least that was my grandfather's argument.
I annoying need to keep Metric and Standard wrenches and such for working on various things. Eventually it won't matter as the US will mostly source parts built over-seas where it will only make sense to adopt metric. And then the Standard system will only be left on road signs and temeratures, oh that's pretty much already happened.
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...