Maybe because many of them realize all our unit systems are entirely arbitrary and are use to converting units already when they do something that requires it.

Maybe because many of them realize all our unit systems are entirely arbitrary and are use to converting units already when they do something that requires it. If the entire world did switch to US imperial, nothing would actually change, you would just learn a different set of arbitrary rules in school. Especially considering how little anyone does advanced sciences without a computer or calculator of some sort these days. Nobody is going to do months of work by hand when a computer can do it in 30 seconds in multiple different ways.