Hi,
Prove it. It is clearly more powerful than a permanently active 3rd level spell that would ordinarily require Concentration (because it doesn't require Concentration). How many other racial abilities are clearly that potent?
Few people on these forums actually prove very much of anything, because that tends to involve math or formal logic.
In this case, your assertion is not quite correct.
1) The ability to cast Fly on someone else is sometimes better than having the ability to fly yourself. So the spell provides advantages that the racial ability does not.
2) The racial ability is conspicuous, even when not used. Sometimes it is advantageous not to be seen as an obvious flyer, until you're actually flying.
3) The racial ability interferes with armor. The spell does not.
4) If you suddenly find yourself in a long series of adventures in which flight is useless, you are stuck with the racial ability. But if you are a spellcaster, you don't have to cast the spell or prepare it; you can even unlearn it if you aren't a prepared caster.
So it isn't *clearly* more powerful. It's up for people to decide. It's subjective.
Also, having a 3rd level spell available all the time or at-will is not necessarily better than having a 1st level spell always available. It really depends on the spell. For example, being able to cast Shield at-will is usually far better than being able to cast Fly at-will. That's also subjective, of course. But Shield is very useful every time someone attacks you while you have an available reaction; flight is great every now and then.
So the comparison doesn't necessarily mean anything. How many racial abilities are clearly that potent? Let's see. A half-elf gets +2 ability bumps and 2 skill proficiencies and can see in the dark instead. That's kind of great. A VHuman gets the feat of his choice at first level, which is something no one else can touch until 4th level; a feat can really pull a character together more than just a clever trick. Dwarves get perfect stat bumps for a cleric or druid, plus other bennies; less sexy than flight but immediately and always useful. It just keeps going. Not all races are equally awesome, but Aarokoa fail at being the only race that makes sense for a Dex or Wis build. That should be a good hint that they might be really good, but are not at all broken.
Anyway,
Ken