Without this detailed biography, we'd all think Batman was a lunatic. Even the Adam West TV show, which was about as edgy as a beach ball in a bouncy castle, went out of its way to reference the Waynes' murder in the very first scene of the pilot episode.
As Batman's opposite, by extension, the Joker should have no clear-cut backstory that defines his every motivation. The best iterations of Joker typically leave his past a mystery, give scant details about it, or make us question its authenticity (more on that later). Joker is nothing but an explanation for his actions. We learn how
Travis Bickle Arthur Fleck is a mentally ill clown who inadvertently inspires a lower-class revolt after gunning down some douchey business bros, and eventually learns that he likes murder so much that it makes him dance over and over.