Explain Like I'm Five | Don't Panic!

We can't make nerves regrow well enough. Regardless of how successful the actual surgery was, the person would be a quadraplegic. Further, the host body has always rejected the transplant. We haven't figured out how to trick our immune systems to accept foreign materials or bodies very well yet. Iirc Russians have tried this a few times with dogs, and I think an Italian doctor tried it with chimps. We can hook up the veins and arteries well enough, but digestive and nervous systems are generally beyond us for now.