Why can't surgeons operate on nerves? We see when operating, surgeons always try to protect the major nerves. Doctors handle bones, muscles, tissues but not nerves. Why? Can they see the nerves?
Well, you can do do various surgical procedures on nerves. You can suture a macroscopic nerve (i.e. a nerve that you can see) that has been injured. However, there are various problems that come with suturing an injured nerve.
One of the main problems is that nerves are pretty unique histologically. Nerves comprise of the longest cells in the body with many of them spanning the distance from the spinal cord out to the fingers or toes. Although these cells are long they are extremely thin. These thin and long cells come together to form a nerve. Each cell is connected to their respective muscle fibers and control their contraction.
When a nerve is injured or ruptured these cells break, and these connections break too. You don't have a lot of time to connect them back together and if you do it is impossible to connect the nerves absolutely correctly together. So although some function of the nerve might be retained it might, at least at first, be severely impaired. This might be somewhat improved with rehabilitation.
For an analogy you could imagine a doorbell with the wires joined into one big cable and then branching out to a bunch of different apartments. If you cut on the big common cable and join it randomly together most of the doorbells might work but would ring in the wrong apartment.
PS: With regard to why surgeons try to avoid injuring nerves that also has to do with avoiding injury to important structures, not just nerves, but also blood vessels, ureters etc.