The gap isn't anywhere near as small as you pretend it is. It's an insult to even mention either Law or Kid in the same sentence as even the old legends, forget the marine freaking admirals lmfao. Kuzan represents the peak of that shit along with Akainu. No way you're seriously trying to sell us the idea that bums like Law and Kid are comparable to that.
Let me put things in to perspective here. Even if Law and Kid were to fight any given admiral in a two-on-one on a fair playfield where things not just magically swing in their favor, they lose 10/10 times.
Why can’t they be? Genuine question
Kid and Law’s last full on screen fight was BM in wano. They only became worth 3 billion after that fight. Then in egghead they were noticeably stronger. Law used a new haki technique and both of them spammed moves that were previously their strongest. Even those feats are outdated as that was last arc.
In any case, they already reached the 3 billion milestone in Wano. Oda isn’t going to have them retread those grounds again. Their next milestone would be 4 or 5 billion, which they will likely achieve by fighting Imu alongside Luffy or a Gorosei.
Plot wise it makes sense for them to be ~admirals, and the BBP/RHP already validated the bounties anyways.