I work as a mechanical engineer and all of the old tooling for stuff is in inches. You don't realize how much stuff you use everyday is designed in the 1950's.
To just re-tool all the fixtures, inspection guages etc. is very wasteful so as it wears out then you try to do new stuff in metric.
There isn't any version of pipe threads in metric that I know of besides o-ring boss ports.
Also to be politically correct, most drawings made are dual dimension (english and metric) if you design it in inches. So what is the big deal?
It is easy for somebody in another country to read dual dimension drawings.
One thing I will say I hate about metric is that if you design a part in inches then the conversion to metric is okay.
If you design a part in metric, the conversion sucks because it is common to display inches as 3 place decimals and mm as 2 place decimals.
This leads to all kinds of arguments during inspection and part approval.