And unfortunately, their operation is thermally based. This means they are inherently inefficient and as you scale up fine control becomes more difficult due to increased thermal mass.
Not to mention it would be susceptible to environmental changes. A prosthetic device would spaz out going from an air conditioned room out into a hot day, for example.
I'm sure there are modest situations where something that can lift 650 times it's weight is more valuable than the thermal limitations. Or maybe super-cold environments?
Pneumatic air muscles are in the same strength to weight range - for example, the Shadow 30mm version is ~470 minimum. They can also be simply manufactured today, cycle very quickly, and are easy to control.