The issue is that the spinal cord is a bundle of cables essentially, a lot of axons from individual neurons. If you sever it, finding the right connection is impossible, so you have to use more blunt tools like electrical stimulation of the whole bundle.

We are getting better and better at labeling individual cells, even at a molecular level. When we understand how to do that, we might be able to do as you propose. I think we will see some forms of paralysis reverted in the coming decades with technologies such as those.