Most of the understanding of biological systems that you're touting ultimately punt their hard questions back to the physicists, so you're really not any better off.
To put it differently, biology can't answer why paraplegics can't walk without first answering how people walk. You can't explain walking if you can't explain motion. Unless the biologists have been holding out on us, they don't have an explanation for motion that would satisfy us any more deeply than you do. Thus, if physicists can't explain something as basic as motion, then biologists can't explain something as simple as why paraplegics can't walk.
To put it differently, biology can't answer why paraplegics can't walk without first answering how people walk. You can't explain walking if you can't explain motion. Unless the biologists have been holding out on us, they don't have an explanation for motion that would satisfy us any more deeply than you do. Thus, if physicists can't explain something as basic as motion, then biologists can't explain something as simple as why paraplegics can't walk.