Why can't we just dump massive currents into spring returning solenoids with ~5mm or ~1/4" range of motion, and amplify that motion through tendon systems for whole joint motion ranges?
Still, I think this idea is under-explored. There are probably applications for robots that move really fast, but only for a second before having to cool down.
Net is that a couple of figures of merit limit the performance of electromagnetic systems, and being sneaky won't let you exceed them.
TLDR; is that you need high current, meaning a lot of ohmic heating. With non-negligible back-EMF resulting in even more losses. Rotating motors essentially "lengthen" the travel of the "plunger" compared to linear motors.