Why can’t a person with a damaged spinal cord make complicated voluntary movements?

Because segments of the spinal cord regulate muscle function; when a part of the spinal cord is damaged, the transmission of a nerve impulse is impaired, and the spinal cord cannot regulate voluntary movements.
Arbitrary movements are acts of a person’s motor behavior. They are carried out with the participation of the cerebral cortex, extrapyramidal system and segmental apparatus of the spinal cord.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.