All true, but in this case airborne have to work in a game - a game that paints history with a very wide brush. The most you can say about any unit in this game is that it gives a flavor of its WWII counterpart. Artillery, for example, are portrayed awfully in this game. They do not pre-bombard anything. They can attack and hold territories by themselves. They do not support infantry at all in defense, only attacking. None of that is historically true, but we can say we have “artillery” and they work in an abstract way within the confines of the game.
If you say that airborne drops must be made only along with land and sea assaults, then you basically make airborne units irrelevant. Why spend the extra IPCs on planes and airborne? Just march the 3 IPC infantry in there w/art support. Or land the troops from the sea along with BB and CA support.
In the case of Crete, there was unexpected heavy defense there - even from civilians. What if in an Axis and Allies game there was nobody at all defending Crete, but a large enemy navy was there surrounding it. In this case dropping airborne would be the only solution. In Market-Garden the goal was to drop a force deep behind enemy lines and hold some territory (in this case bridges) until the land forces caught up. the goal was to accelerate the allied advance into Germany. That is basically what you would be doing in Axis and Allies - drop a relatively small force of airborne 2 spaces inside enemy territory in a place they don’t expect - perhaps on an enemy factory or a key canal or strait, with the goal of accomplishing some objective.