Money.

Because it's a massive waste of money for little to no benefit, and barely actually comes up because unit conversion is trivial and is done constantly regardless of overall unit system.

Armchair unit system fanatics make it out to be such a bigger deal than it is. Whether im working in metric or standard I'm doing several to several dozen dimensional analyses anyway, normally with industry specific units. Which again, exist in both standard and si.

when i have to convert cm to m I move the comma

I feel the real war we need to fight over is whether the decimal point or the decimal comma is the one true decimal separator.

How is it hard? Aren't people in the rest of the world much better educated than people in the US?

Freedom units.. Oh, you mean imperial? As in, Britain? Yeah, we're just using their old system and haven't bothered to change unless there's a practical necessity.