Why is it when a comic book superhero that just has similar powers to Superman already called copycat?

This premise is not entirely accurate. There are many superheroes with powers similar to Superman, and they are not all necessarily considered copycats. Characters like Thor, Wonder Woman, and Captain Marvel have all been around for decades and have their own unique fan bases and stories. The idea of a copycat implies that a character is directly copying another character, which is not always the case.