I'm interested in the defense of the rule. Do you think it's more realistic to have the AoO coming into a threatened square or retreating from it?
I think that there is some merit to both.
Retreating is harder than either advancing or defending. Given the turn based discontinious nature of the game, the simulation of this is easiest done by controlling how you may disengauge. Note that you may take a disengauge action to negate the attack of opportunity and you may under the official rules at least move back with a 5' step, so the existing rules only penalize movement which does not seem to involve either attack or defense.
It's possible to take a feat where by those that enter your threat zone while taking the charge action draw an AoO. This forces the target to engage more carefully. Likewise, reach weapons simulate the advantage of a weapon with longer reach. But there is also some subtlety in the rules because you don't want to close with the target in such a way that you are subject to counter charging. That is to say, if you can't double move and attack, if you advance then the target CAN double move and attack you. In this way, you draw an attack by advancing on the enemy. Mechanically it is very difficult than what the OP suggests, but in terms of attacks exchanged it is quite similar.
Of course, this doesn't need to be perfectly realistic. But...
If I'm fighting Mike Tyson, I think it would be hard to even approach him and begin the combat. But, if I were already in the combat, it seems a bit easier to retreat and put distance between us.
Have you ever done much fencing? That first step back is easy and a good and perhaps essential defensive measure. If however you take that second or third step back, you've yielded the initiative to your opponent and your defense gets progressively harder until you do something to threaten your foe enough to get them back on their heals and allowing you to step up. Going backward is just harder in many ways than going forwards.
If the d20 rules allowed for an AoO as someone ENTERS a threatened space, then fighters would dance around each other, just out of melee range, looking for that particular hole in the foe's defenses in which to strike.
My assumption is that this is exactly what is happening within the abstract 6 seconds (or 30 seconds depending on your system) of combat in a round. This is not a one blow/one parry per turn system. This assumes quite a bit happens in a round. Fighters are dancing around and probing each others defences in D&D. But often as not untrained fighters do that because they think it looks cool, or because they are nervous or afraid. It's not really an essential aspect of a fight except in the movies. Two Navy Seals certainly DON'T fight like that. Boxers sometimes do that depending on their style, usually because one of them is not as physically strong as the other and can't afford to get in the clinch for fear of being worn out. Instead, they are trying to wear out the opposing boxer who is often heavier by forcing him to move. But I don't think you should mistake a dodge, a defensive stance, or combat expertise in action for the idea that the fighters are 10' away from each other waiting for the other to close to punching range. No, the fighters are like 5' away from each other, dancing in and out of punching distance neither drawing an AoO from the other but rather engaging in what must be seen as their regular attack reutines.