Why do crushed or cut nerves take so long to heal (months or years)?

Unlike some other animals, human nerves undergo a process called Wallerian Degeneration when injured. This means that after the injury to the nerve, the nerve endings will shrivel up and degenerate all the way back to the site of the nerve.

This process takes a long time because after degeneration the nerve must regrow all the way back to the area of injury like a vine growing back up a wall when you cut it.