Easier said than done. 
Firstly, not every one of your airport cities are going to have factories and power plants (where you would rushbuy) and your factory cities may not have airports. Secondly, using the whip is not ideal in many cities for many reasons including happiness issues, city size, not owning Kremlin etc. Thirdly, what if all your units (or at least most of them) are coming from one city and your others have more important things to build? Fourthly, what about the units you have used in a previous battle, sitting in a pile in the middle of your empire? You can't just leave them behind if you're really going into a tough battle (the war is not won yet). I could list other reasons but in most games I play it's never as simple as rush-buying units and simply airlifting them the next turn.
In an ideal world there'd be a button that you could press when you had a stack of units selected, sending every one of the units to nearby airports and ordering them to wait once they reach the airports, so the next turn they can be airlifted.

Firstly, not every one of your airport cities are going to have factories and power plants (where you would rushbuy) and your factory cities may not have airports. Secondly, using the whip is not ideal in many cities for many reasons including happiness issues, city size, not owning Kremlin etc. Thirdly, what if all your units (or at least most of them) are coming from one city and your others have more important things to build? Fourthly, what about the units you have used in a previous battle, sitting in a pile in the middle of your empire? You can't just leave them behind if you're really going into a tough battle (the war is not won yet). I could list other reasons but in most games I play it's never as simple as rush-buying units and simply airlifting them the next turn.
In an ideal world there'd be a button that you could press when you had a stack of units selected, sending every one of the units to nearby airports and ordering them to wait once they reach the airports, so the next turn they can be airlifted.
