Well, those things may save you from an opponent whose movement speed is 30 or lower, it doesn't offer any protection against things that have a movement speed of 40. And while it does work relatively effectively against the Iron Golem in your example, I imagine any circumstance in which you are going to be fighting an Iron Golem, I feel one should also note that the locations to which you can move exactly 40' away and still have a clear shot are going to be limited. On the other hand, an Arakocra is simply never going to run out of sky so long as you are outdoors and thus will always having a clear shot unless the enemy does and can hide under good cover.
(1) If you're outdoors with plenty of sky for the Aarakocra, it's probably also good terrain for the Mobile human too.
(2) You can attack the Iron Golem
before you move (e.g. with Booming Blade), so you're not constrained to only locations where you "have a clear shot" after moving. (And besides, so what if you don't get a shot off every single round?)
(3) Nitpick: if you move only 40', you are dead. You need to move 45' to wind up 50' or further away from the Iron Golem, or it can kill you.
(4) If you really want to kite, you can always combine Longstrider + Expeditious Retreat, for 100' of movement after a Booming Blade. That lets the first-level Mobile wizard kite even the fast creatures like griffons--something which a typical Aarakocra cannot do. As I said, better in some ways, worse in others.
It is one thing to say "my mobile character can infinitely move out of range while still blasting the slowest creature in the monster manual" and quite another to say "regardless of how fast the opponent is, it can literally never catch me and has to rely on any ranged attacks it might have at its disposal".
Iron Golems aren't the slowest creature in the MM. (That might be the Gibbering Mouther.) They have a fairly typical speed. Most of the really fast things have a flying speed as fast or faster than an Aarakocra anyway, so the overall effect is similar: if the DM builds a standard adventure expecting a melee-centric party, it will fail in approximately the same way for Mobile humans and Aarakocras. And most of the DM-side countermeasures against one work against the other: cage matches, locked doors, ranged weaponry on opponents (goblins, gnolls, hobgoblins). If you've built your adventure to be secure against Mobile humans and Mounted Combatant humans, it's probably going to work fine for Aarakocras too.
The Aarakocra, for example, even at level 1 presuming it has enough arrows, can eventually kill a CR14 Ice Devil whose movement speed is 40 without the Ice Devil being able to do anything in retaliation. Even if it takes 180 rounds to do it due to the Ice Devil's damage reduction, the Aarakocra has a guaranteed victory.
How's that? The Ice Devil can just form a dome over itself, or a wall for cover.