When someone is injured, whether it's a broken bone or a laceration, the body goes into healing mode immediately. Bones will grow back together and new cells will form on the skin to heal a cut. The same is not true when it's damage to the nerves. Bones that have been broken do heal, but the site of the break is always damaged to a certain extent. The same is true of cuts on the skin, the cells that make up the tissue that heal the skin are not exact copies of what was there before.
When it comes to nerves, the cells in the central nervous system are highly specialized. If there is damage, growing different cells at the site of the damage is likely to cause permanent effects. It's similar to when a light bulb goes out. It can be replaced, however the same size and type of bulb is required, and the body doesn't make exact copies of nerve cells. While some regrowth happens, the circuit in the nerve pathway will likely always be damaged, to a certain extent and this can leave a person with motor problems or cognitive impairment.