Here's why:
* Imitate means to copy someone or something.
* Alter means to change something.
While they both involve changing something, they are distinct concepts. The antonym of "imitate" depends on the context:
* If imitating means copying exactly: The antonym would be originate or create.
* If imitating means trying to be like something else: The antonym would be remain or stay the same.
Example:
* Imitate: The student tried to imitate the teacher's writing style.
* Alter: The tailor altered the dress to fit her better.
The opposite of imitating the teacher's style would be to create their own unique style. The opposite of altering the dress would be to leave it unchanged.