A familiar is squishier and has much less utility and ability than a PC. Basically no ability to end the scenario there even if conditions favored it/were non-hostile.
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But a stealthy rogue or ranger...
You can totally stealth in flight.
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...and wear armour... An unarmoured PC...
You can totally wear light armor in flight as an aarakocra at least, so this is a gross mischaracterization of RAW. Nothing says you couldn't even have a shield. So AC is not a problem.
If you're flying out of range of most (why not any?) ground attacks, unknown terrain isn't a threat until you have to go in for your target. At which point you stealth. At which point you have armour. At which point you have the utility of a full PC. Just like normal scouting, except you got there way faster and can get back way faster.
And to make that not viable, basically what I'm hearing is you end up having to target the PC with combat. Which means you end up having to insert combat into your game, everywhere. Somehow. Even if there wasn't "supposed" to be hostile forces at the destination. Even if you are trying to run a low combat game...
...
I think we can do this all day. Which is, again, why I don't think it's too fruitful to dissect specifics. Specifics are just one instance. The instances are unlimited. Flight literally adds a whole new dimension to the game. For one player.
And I think you don't see multiple aarakocras likely has just as much a combination of a lot of ppl don't like to double up on character concepts (which is to say, they have an abundance of things they want to try) and simply a lot of ppl don't seem to care for anthropomorphic PCs as much as "the classics".