Short answer: Magic.
I came across this video today of Todd Kenreck from DndBeyond interviewing Jeremy Crawford from WotC about the difference between Arcane Magic and Divine magic and he gave a great answer to this question at 0:30 expressing that, in our world, dragons couldn't fly, but they can in the D&D multiverse because of magic.
The long-short ...
Have you ever noticed how a flying animal's wings come out from their sides?
Every illustration I see of an aarakocra places their wings at rest a slight distance to each side. In normal use, their wings should never need to extend far enough back to touch each other, and the use of a backpack shouldn't inhibit an aarakocra's attempts to squeeze into a ...
Why does it have to be a "back" pack?
Before now, I've taken backpacks for people who've hurt themselves and needed help. Since I've been carrying a pack myself, I've put their pack on my front. I can report that the result is well-balanced, fits fairly well if it doesn't have a structured back, and causes no problems at all. The only issue for carrying ...
Presumably, their backpack is designed so that it allows for flying.
The only restriction the Aarakocra fly speed poses is medium or heavy armor.
You have a flying speed of 50 feet. To use this speed, you can’t be wearing medium or heavy armor.
So as far as mechanics of the game go it is possible.
They are an intelligent race with a society so they ...
Yes, I believe it does allow you to change into an Aarakocra.
Alter Self (transmutation)
Aquatic Adaptation. You adapt your body to an aquatic environment, sprouting gills, and growing webbing between your fingers. You can breathe underwater and gain a swimming speed equal to your walking speed.
Change Appearance. You transform your appearance. You ...
It doesn't break anything, but it is a misuse of the spell level
Allowing a flying steed for Find Steed breaks the balance of the 4th Level Find Greater Steed. A Paladin has to wait until 13th Level to get Find Greater Steed, but in this case they get the flying aspect at 5th Level.
It isn't any more broken than giving the Paladin a Broom of Flying at ...
Chapter 5 of the Dungeon's Master Guide presents some options
The Wilderness Survival section in the Chapter 5 of the DMG presents rules for strong wind conditions (pg. 110). It is as follows, emphasis mine:
A strong wind imposes disadvantage on ranged weapon attack rolls and Wisdom (Perception) checks that rely on hearing. A strong wind also ...
One option is that the DM might determine the storm to constitute difficult terrain; then the fly speed would be halved.
You move at half speed in difficult terrain--moving 1 foot in difficult terrain costs 2 feet of speed--so you can cover only half the normal distance in a minute, an hour, or a day.