Because they can, and because it raises the cost of the system by at least one zero.
I had an encounter with the medical tech prices when rehabilitating my broken shoulder. The assistive rigs were up to 100 times more expensive than what I'd consider a reasonable cost. Made me thinking about something like Thingiverse, but for designs of medical/assistive/rehab tech; so in case of need there is no reason to go the Approved Way and shell out money that aren't available, but just visit your nearest locksmith and get it cut/welded together and get the motors and microcontroller attached. Or get the parts 3d-printed, possibly customized to your own body by entering either the measurements or even a 3d scan of the person or body part.
The Healthcare Industry won't like it, but we wouldn't have those $1-a-pop tracks on iTunes without Napster too. So don't ask for approval and roll it out, possibly in some obscure jurisdiction outside of the Big Med lawyers' reach.