Why do humans say "How" are you?

I understand, darling. YOU do not understand. "How" makes perfect sense in those circumstances. According to the OED one of the meanings of "how" is "in what condition or state."

"In what condition or state are you?" makes perfect sense.

Anyway, words mean what people understand them to mean. Every English speaker (except you) understands what this means. You might as well argue that "cat" is a stupid word because it doesn't mean "cat."

And who the fcuk is somebody calling themselves "Qroen" so question whether a word makes sense? "Qroen" makes no fcuking sense at all.

When someone is asking "how are you?" S/he wants to know about your condition. So it absolutely makes sence.

When someone answers the question "How are you?" they describe HOW they feel.
Ex: "How are you?"
"I am fine." (This person describes how they are feeling, not what they are feeling. Fine is an adjective, not a noun.)

how (adv) - To what extent or amount or degree.
... "How are you ?" = “To what degree on the continuum of health are you ?"

"How are you?" = "How is your health?" = (by implication) "I hope that you are in good health".

"Who are you?" would be a strange greeting to say to somebody you know!

I can't account for humans or for primates in general.

Sounds better than "What are you" ?

It's an idiom - an expression in speach that has a meaning that is different from the meanings of the individual words.

"What are you?" makes no sense at all.

How means: do you, and are you means: wanna bone?, so do you wanna bone?, and if you reply, and it proves you are good dog.