Why do we say "what's up?" to people?

Unless someone is upset and “whats up?” is used to mean “what’s wrong?”, then usually there is no response that really fits “whats up?” Am I the only one that notices that this is sometimes awkward? Coworkers, friends, family, acquaintances, bosses, classmates, EVERYONE asks “whats up?” But do they really want to know whats up in your life right now, or do they want to know how you are…ugh, I just wish people would stop saying “whats up?” to me. “How are you?” would suffice, and much better if i do say so myself.

Like for example, I’m at work tonight tryin to load some boxes into a truck and someone I don’t know barges in to help. “whats up?” is the greeting. NOW I WANT TO TALK TO THIS PERSON IN A FRIENDLY MANNER, but what the hell am I supposed to say? “Oh, just loading some boxes” sounds dumb as hell, and is what I usually sheepishly say and “not much” is plain boring and makes you seem uninteresting.

I know this is not a big deal but I’m just wondering if anyone else feels this way about this phrase.