The reason is that, in the middle of the hand, it may be imperative to cash your defensive tricks quickly—before everything goes away on something Declarer has set up, or is about to set up. SO, you are expected to lead the ACE from Axxxx and get a signal from partner as to whether to continue THAT suit or try the only other suit that would be left for possible defensive tricks. So Ace canNOT guarantee King as well. (And logically, if you were on opening lead and led something else earlier, you probably DON'T have both Ace and King or you would have led that suit in the beginning. This additional logic does not apply if you were not on lead, of course.)
Thus, when the KING is led, in the middle of the hand, it can promise, either AK(x)(x) OR KQ(x)(x), and partner is supposed to work it out.