Because both characters are really good and have effective tools against each other. In high-level games between top-tier characters you'll frequently see both players doing extremely "unfair" and "broken" things to each other, but the fact that both players can do them means they compensate for each other over the course of the game/set and the matchups end up relatively balanced overall. In addition to this, the options you're talking about have effective counterplay despite how good they are, which prevents them from being "too good". Because those tools can't be dealt with as well at low levels of play, they have more of an impact there (and seem much stronger), but they can still produce seemingly highly imbalanced results at high level.
It's most obvious with the punish game (but applies to neutral too): the effectiveness of the punish games both characters have means that comparatively fewer interactions are needed to win than between worse characters (or less effectively piloted forms of the good characters), which leads to more extreme outcomes. A simple way to visualise the concept is to assume that each player will win neutral about 50% of the time; if they can zero-to-death each other every time they do so then you're going to end up with a lot more games where one player dominates than if they can only get small punishes.
If you happen to play any trading card games then you'll know they demonstrate this quite clearly*: because of the power creep, where the overall power level of the top decks invariably tends to increase constantly as new cards are released, eventually the old decks/cards become so much worse than the new ones as to be completely unviable. And yet despite them being broken as hell by the old decks' standards, the new/good decks are still usually competitive against and roughly even with each other. It also tends to lead to an increase in the speed of the game and a reduced tolerance for misplays as the ability to take advantage of an opening increases, in a way which is parallel to the impact increasingly optimised punish games have in Melee.
* Yu-Gi-Oh is the best example I know of, between fundamental attributes of the mechanics, the fact that all cards remain legal at the maximum quantity forever unless explicitly/individually made otherwise, and the development team being really liberal.
