Wrong.
The schematics for the motherboard and any other boards (like the LCD connector board) can be open or closed. Also, the CAD files for printing the case components on a 3D printer.
Hardware absolutely can be open-sourced.
You might disagree with the definition, if you somehow consider all the information that goes into the manufacturing of a physical object to be something else than the "source" for taht object, but this is how the term is being used by a lot of people, who are actually doing this.
I think it was Ed Wilson who pointed out the 1d DNA leading to 3d structures (perhaps Schroedinger in his book What is Life or Gould in one of the many essays). I can track the reference down if needed.
Meta: I think you are wrong, but I'm surprised at the degree of down-voting.
But, how does that work with hardware? How do we trust all the chips? How do we know there isn't something iffy built in at manufacture? Then, how can those same cleverer than me people discover it? How do they inspect the encased silicone circuit?
So, in that vein, I see the point you might have been making, and actually, I think its a worthwhile question. Because to me, it does look like what you describe, a sort of closed source hardware which runs all open source software. And I personally dont know that that is safe. Or is it?
You can't really trust silicon, and this leads to a larger point about much of what has been called open-source hardware: many of the open-source boards out there which are really popular are little more than reference designs translated from a datasheet to a schematic capture program for a closed-source blob of silicon. However, this point doesn't escape many people in "the movement," the problem has just been that the technology that meets the users' expectation of performance and capability is out of their financial reach to duplicate. So, the users settle for "re-create" when they can, rather than "trust at all levels". In the majority of cases, absolute trust is not required, because a breech of that trust can result in practically nothing. ("Oh, this AVR chip is back-doored, allowing you to access its 32k of flash, but I haven't provided any means by which a 3rd party could access that via hardware.")
Many of the security flaws and known back doors are in the firmware, (see routers for example), which Bunny is resolving by making Open Source. It's probably impossible to eliminate all attack vectors, but by opening the processes up, you can eliminate most of the easier ones.