And all that training and calibration requires highly-specialised workers to be able to help you. So it's more than just the cost of the device itself. Even if you could mass-produce them, I imagine they'd still need a lot of tailoring to the individual.
I actually saw someone using a motorised prosthesis for the first time recently, and it was very impressive to watch. I'm optimistic for the future, I just hope everyone who needs such technology can get access to it.