One of the things that makes it exciting to get hardware mass-produced is that, once you've designed and built your prototype and you send the design off to a manufacturer, you find out that they won't make what you specified. They'll tell you that certain components have cheaper equivalents, some things are too close together to be easy to assemble, some components aren't available in the quantity you'd need (or are going to go out of production). It's like if you built some software, and then tar decided you have too many zeros in your executable. If you've got a good manufacturer, they'll be right about exactly what's wrong with your design (for production right then, because it will change) and what they can do to fix it. If you don't have good communication with the manufacturer, they'll just make the changes.
Of course, if you're doing anything more clever than they understand, your devices won't actually work. In general, you've specified some things more exactly than necessary (e.g., you have to specify a part number for your LED, because half of the LED brands need to be installed in one orientation and half in the other, so it won't work to say "random LED, oriented like this"; but it does work to switch brands and install it backwards); but other things need to actually be exactly like you say.
And, traditionally, you're trying to keep other organizations from being able to figure out your design and release something similar, so you've obfuscated your design; and you don't trust the manufacturer and you're half pretending that they aren't reverse-engineering everything you've done so they can redo it for you, so you're not going to give them all the important information.
Now, if you've got an OSH design, you obviously aren't trying to make it hard to reverse-engineer, and you've got all of the information in the design. You've probably got a page of schematic for each major part with the components that are for it, and the manufacturer can just look up whether they can swap values and what tolerances actually matter. If you have the relationship in place early, you could probably even avoid doing the work they're just going to redo anyway. You usually think of open source as allowing other individuals to modify your code and run it, and hardware isn't so much like that because there's a substantial cost to producing a working device from a design (if I change the design of my cell phone, I have to spend a ton of money getting a new cell phone built to my new design). But there is the aspect that the people producing your physical objects will do a better job if the source is available, in a way that doesn't apply to people pressing CDs of software.