Why is that? Common components use thous but tooling moved to mils?
Most parts were imperial IIRC until at some point they started selling parts that were too small (smaller than 0201) and switched to metric. I can understand wanting to switch to metric, but reusing the same goddamn size codes is too chaotic for my taste. Come up with different numbers.
You'll still use metric, but you'll just end up seeing lots of 2.54 or 0.254 everywhere.
My unsophisticated wild-ass guess about why they persevere: Like Fahrenheit, it so happens that mils as a scale intuitively correspond to sizes of trace we care about. A 5 mil trace is pretty run of the mill for signals. 20 mils for most IC supply traces. 3 mils is small and expensive to make (last I checked! capabilities have changed rapidly in my decade of experience)
I think the intuition argument is often just a post hoc assumption for things that we are actually just familiar with.
Thinking about it a little harder, I think the reason is simply institutional inertia, same as for every other industry that retains inch-pound-second based systems.
The numbers get even better on the metric system. I was expecting them to just not get much worse, since they were picked on the imperial on.