Why do people call person a vegetable?

Answer 1

A person who has had a brain injury who is awake but not consciously aware is called a vegetable, because he or she is unable to respond at the basic level of an animal, much less with fully human consciousness. The formal medical term is a person who is in a "vegetative state."

Answer 2

Vegetable is sometimes used for persons whose mental capacity is so reduced that they might as well be vegetables. They only sit there, doing nothing apart from slowly getting older.