Whilst it theoretically makes more sense to pilfer stones from the base of the pyramid than from on the higher courses, one must keep in mind that without 500 exuberant slaves pulling on ropes to assist, the average Arab pilferer probably only had a donkey with which to lug his daily scavenge back to Cairo.
Now, if I were a poor, old donkey faced with lugging the lower stones, or even those, just above, which are smaller but are by no means small, I might become a tad stubborn. Either that, or buckle at the knees, or worse, thus becoming an ingredient for the night's tagine depending on my master's level of displeasure.
Maybe those stones higher up were much smaller.
Anyway, if they were still in place, the apex would probably not be in danger of collapse. So, whoever was responsible lacked any commonsense.