I think the most important point here is correcting what you said about any pocket pair being equivalent to KK. Let's assume Villain is shoving AT, AJ, AQ, AK, AA. This is his shoving range. For completeness, let's look at the odds
of him having any of these hands.
AA - 6 combos
AK - 8 combos (you have 2 of the kings)
AQ - 16 combos
AJ - 16 combos
AT - 16 combos
Total: 62 combos. Which means he only has AA about 10% of the times he shoves. And this is assuming we know his shoving range is this tight (in HU it might not be). So with KK, you are dominating his range 90% of the time, since as you said, he pretty much has to hit an A when he doesn't have two of them already.
Now let's consider 99 against the same range. Now, he's still dominating you the 10% of the time he has AA, but what's happening the other 90%? It's basically a flip. Against each hand in his range (aside from AA), you are winning about 55% of the time, and he is winning about 45% of the time. Because he doesn't have to hit an ace to beat you
. If the kicker to his ace is 10 or higher and you have 99, he can just pair the kicker to be ahead. With AT he can hit an ace OR a 10, with AJ he can hit an ace OR a jack, etc etc.
I think some of the useful posts by Big_Rudy and MediaBLITZ are getting lost amidst the "LOLZ WHY WOULD YOU FOLD KK PRE?!??!" Make sure you understand why
this is never a correct fold, not just the fact that it isn't.