Yes but as OP pointed out, that requires you (or them) to spec it all out. And if you do, that’s how it works, and that’s probably exactly what happened when you buy any well-made product that was designed in America and built in China.
But a whole lot of manufacturing relies on the OEM not just for the production but also some of the engineering. They probably don’t communicate to you all of the little parts you didn’t spec because they don’t think you want them to.
A friend of mine worked for a major vacuum company that has 3 well-known lines. The very high end line is manufactured in the USA. The mid range line is fully designed here but manufactured there.
The low end line they basically tell a Chinese OEM what it should look like and certain parts of it and let the OEM fill in the blanks. OEM makes vacuums for other brands too, they know what size wires to use where, why waste money on American salaries speccing all the tedium?
OP is making something fundamentally different than what exists in some ways. It’s a lamp, but obviously it has some considerations other lamps don’t.
Also hardware (as OP pointed out) has long development cycles during which prices can actually fluctuate significantly. You don’t want to lock in cost plus pricing or you’ll end up buying components from their sister company at astronomical prices. You might not have time to source 100 different parts from Chinese OEMs yourself. Etc.
The whole thing is a complex guessing game. I’ve only manufactured approximately the simplest objects imaginable (stuff like gaskets and cups) and found it to be far more complex than I thought it would be. Manageable but something like a lamp I could see being a lot of unexpected work. I did not see, for instance, how much effort I had to put in speccing out the packaging for simple objects but then I found myself repacking things here.