It's a relatively minor thing they left out of the game. Historically, air dropping supplies could never have been a reliable way of transporting large amounts of them. Air supplying cut off divisions for short periods of time out of respect for the lives of your men and the importance of their objective *ahem* *Bastogne* is another story.
The big deal is that IRL it costs TONS of fuel to fly supplies for an entire army. Not in this game though, due to the absence of used fuel, which is why it would be abused. The major reason I see for the devs leaving out the possibility of flying supplies for your entire army is because of the logistics system present in HOI4 in general. I don't see a reason why flying supplies should be completely left out, but it would need to cost a bunch of fuel so as to make it horrifyingly inefficient to try and do on a mass scale. HOI4 does not support unit fuel usage, unfortunately. Hopefully someone will fix this
Of course it should be balanced accordingly. For these very reasons.
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