"The principals that these instruments operate on are not 'rocket
science', I will agree, but the skills required to build them are
rare.

I would NEVER contest that!

In truth, if you took 20,000 average people, I bet not over 100 of
them could operate a lathe. Three might fix a watch. Three might be
able to build a reflector telescope. I doubt that you could find 1 in
20,000 that could build a lathe or a milling machine that was
reasonably accurate (a good welder might make some kind of servicable
tool). Especially if no working lathes and milling machines were
available to start with... But a gravity meter??? This is not your
usual instrument.