In English, it is used more as a greeting, you don't really expect an answer. Most of the time, people will respond with the same question. In England, it's very common to greet people with '(Are) you alright mate?' (mate is British slang for friend). It has the same meaning as 'how are you?', but again, it is used as a greeting, often instead of 'hi' or 'hello'. It is very informal, but an extremely popular phrase in the UK. (Note that the word 'mate' is not used in this way in the USA and to ask 'are you alright' in the USA would imply that you think they have a problem.)

Same thing happens in Spanish and Portuguese—I'd dare to say that it happens in all western languages. And, to be honest, I've wondered about the same thing. The answer might be more complex than just: "because, that's the way it is". It's almost as if a bit of hypocrisy is necessary to help padding social interactions of everyday life.

thanks for your answers)

you know, i think the questioner him/herself knows that in most cases the answer is just the same: OK. but it's just a way to show that u care about that guys health, feelings and so on. that's the motive for asking this question, i think. i hope i could help! :)