...we live in the age when the built up wheelset from an online seller
is available cheaper than you can buy the individual parts, primarily because
of economies of scale and the invention of the wheel assembly machines that
tension everything simultaneously.
Machine built wheels are a nice starting point, but so far do not approach
what you are able to do yourself in tensioning evenly at the higher tensions
required for a durable wheel. Also, I'm pretty sure they don't stress relieve 'em.
So I guess maybe you could make a side income tensioning and relieving machine
built wheels for folks who want them to function well and last longer.
And some guys really do want wheels built to order that you cannot buy prebuilt.
Buy otherwise, I think you are correct. It's not something I'd try to do for a living.