No need to get salty. The fact is a humanoid is a humanoid. It is bi-pedal. That is, "two-feet." The fact the aaracokra has 6 "limbs" and so does a centaur, does not make them the same. A Humanoid is a biped. A centaur (as any four-legged creature is) is a quadruped.
The answer to your question, "hiding behind a bush" similarities notwithstanding, is because the centaur does not walk on two legs like a human, hence not "humanoid."
The mermen are considered humanoid, I suppose, because they have "less" than two legs, not more. So humanoid is the simplest/most convenient place to put them.
That is not the case for centaurs...and the fact they are not humanoid does not make them "unplayable." The fact they would have significant difficulties navigating most subterranean terrains, climbing of any sort, tight urban (or any other setting than a barn, I suppose) buildings make them a difficult sell for most campaigns/DMs.
I, personally, have/allow centaurs as PCs in my homebrew world. But they do have trouble/find themselves in difficult positions or separated off from the group from time to time. They're just difficult to get around unless you're going to spend the whole time in fields and hills and woods.
