if you want a thematic explanation, flying creatures aren't heavy enough for charging the enemy to increase their melee damage output. How do you think angels fly? Magic? And what does the magic do to make their flying possible? Why, it lowers the force of gravity on them of course! And if you argue that the force of their charging attack would be equal to their mass x their acceleration, and not their weight x acceleration, I would then say that they're lowering their weight and increasing their inertial mass, and therefore lowering their acceleration. So they would have so much inertial mass that charging would hit really hard--if they could actually hit at all while charging that is. Their great inertial mass makes it much harder for them to move normally. They use the light of their holy spirits to command their wings to carry them, and their light is so great that it's much more likely to move them longer distances rather shorter ones. Hence why they have no problem moving between zones while flying but can't increase their melee attack by charging from the air. They would have to descend to the ground and maybe retract their wings first, like when an airplane has to go down a runway when landing because of how fast it's going.
And if you tell me that such a process of flying using the light of one's holy spirit requires infinite energy, well, they ARE quite holy. The light of their holy spirits are pure and infinite. Their souls generate that infinite energy, and since energy in Etheria doesn't always need to stay in the form of quantum packets, they can fold that infinite energy into a fractal that fills a finite amount of space, such as their bodies.
Of course, dark mages sometimes have a similar process. The fear, hatred, and evil in their souls are, while not always infinite, they are very big, but don't necessarily have to be in quantum packets. Therefore, they can be folded into a fractal to fill a finite space, like a phylactery.
In summary, the reason (IMO) that fliers can't charge into their zone is because flying is, at least partially, an inherently magical ability that uses the flier's own brave and unbounded will to keep them aloft, and that brave and unbounded will represents a large amount of energy that increases their inertial mass. decreases their weight, and allows them to literally defy gravity.
If you think that this explanation is unnecessarily complicated, now you know a very good reason for why people who live in a magical world usually handwave unexplained aspects of magical phenomenon just by saying "It's magic!" Magical Mechanics are extremely complicated and difficult to study, as any sortilegian wizard would tell you.
