i've worked in a few factories and this is not always true, especially with short runs.

to make a machine assemble a thousand things you gotta "tool up". that used to mean designing and building the tool that would do the repetitive motion but nowadays its just as much laying out gcode as it is figuring out how to make the more generalized machinery perform the specific tasks required for putting together some thing.

so take a computer mouse, there's like four parts. a usb wire, a circuit board, the bottom and the top. assembling the mouse is plugging the wire into the circuit board, aligning the board to the standoffs in either the top or bottom and make sure the wire is going out the hole then snap the top or bottom to it's counterpart then test.

probably fifteen seconds from parts to tested and ready for packing?

so in a thousand unit run you're looking at four and a quarter hours of human work. lets go ahead and round up to five, since someone is gonna have to set up our mouse assemblers bench, write out instructions, unpack the parts and dump them into bins, etc. it won't be 45 minutes of work, but more slop is better!

so for a thousand unit run you could pay your mouse assembler $15/hr and still only have 7.5c unit cost of assembly.

packing is another one that often gets done by people, but a mouse is pretty much wrap, tie, bag, box. maybe another fifteen seconds of labor, so add 7.5c onto your assembly and youre looking pretty good.

now your contract factory isn't gonna quote you what they think they can hit, they're gonna drag their laziest, slowest worker over to do the process five times, take the average and quote that. then they can charge you for ten hours when it only took five and pocket the difference. even then 30c per unit is most likely less than the robot equivalent.

just the cost of a quote to tool up for that run is maybe $50? free quotes weren't the norm domestically back in the day, but they were becoming more common overseas. then you've got the cost of the tooling (we'll keep ip like part layouts and gcode here) and the machine time itself!

there's also the actual injection molding of the top and bottom, making the cord, assembling the board, etc, but thats a whole nother conversation!