Metric fasteners in the US cost significantly more than their inch sized counterparts. Same materials, the tooling exists, the price is higher.
I've not found an anti-roll bar on any car from Europe, NA, or Asia that is manufactured to a metric size. Yet all of them specify the size in millimeters...gotta love the 25.4 millimeter bar.
I guess all engineers agonised for a term or so during their first undergraduate year coming to grips with 438,972 different systems of units of measure
Past first year, still find it outrageous and annoying.
The PS/SAE HP/DIN HP/CV/whatever stuff is not my idea of fun. Oddly enough, I haven't yet found the watt that isn't a watt. Am I not looking hard enough?
This idea that the units don't matter so long as the system is consistently applied also seems a bit weak to me. It is very easy for me to remember how the watt is defined. Its easy for me to relate it to units like the volt and ampere. It is considerably less easy and less accurate for me to do such mental gymnastics with whatever system of units. It would appear to me that the SI system and the technical units related to it are just plain better than the alternatives.
In any case, even if one doesn't care to admit that one system is inherently better than another, surely its obvious that multiple systems in use
will cost lots of cash and time in order to avoid safety and quality problems.