Gammaradiation sterilization is used for simple and small medical tools and gear - scalpels, syringes, gloves and the like. It's impractical for bigger pieces of equipment. That wouldn't be a problem if you first irradiate exposed components, like tubes, straps, and so on, since the housing of the breathing apparatus, pumps etc don't have to be sterile since they have no contact with the patient, you'd still need to assemble the entire thing under sterile conditions, by people who know what they are doing. One careless touch and you could kill someone. A flake of skin, a piece of hair, that can be it. I know, this sounds dramatic, and the odds are minimal, but it's usually not a chance we're willing to take.
I'm not saying I don't think GM or Ford aren't capable to do it, but there is a reason why large manufacturers either don't bother or create a substitute firm to produce medical gear, it's a pain in the ass with a lot of scrutiny. Not saying cars don't require quality work, it's just not as much of a hassle.