My guess is that unless the manufacturer wants to buy them premade in bulk, it costs too much to make them themselves. They would have to create another department, hire the people, all the overhead crap. I don't think you'd like the added cost a manufactuerer would place on a product that used a specially designed power cord.
They could enlist someone into the DIY PCs to make them for them, but that is another level of labor and management too. I do agree, it would be nice!
cheers,
Dick