how are you doing?

"I'm sooooo tired!" This is the response I get about 35% of the time when I ask someone how they are doing. The other 65% the response is "Good!", regardless of wether they really are. Ever had someone walking past you very quickly and they throw it out there and don't give you the time for a response to how you're really doing? I realized long ago that they don't really have the time/interest to know how you're doing, they just wanted to acknowledge your presence, which is fine, but why not just say "Hi"?