because legacy heroes are conceptually lazy so nobody competent ever wants to write one. "It's a different person but they have the same powers/look/villains/themes" is not something anyone with talent actually wants to write. As a result unless the legacy character is a legacy in name only (Ms. Marvel) or comes with a pre-existing excuse (Ghost Rider, Nova) legacy characters are universally terrible when they first start.
Later on - five years or a decade down the track when these characters have had a while to soak up time and are dissociated from the characters they originally leeched off more competent writers will pick them up and make them more interesting. Then eventually after a few decades of this those characters become characters that just exist, like the various Green Lanterns, Flashes, and so on.