Conversations or not??

Here I am in Iraq, walking to work when I pass by someone who says, "How's it goin sergeant?" I reply with, "Not bad". Pretty normal right? That's what I thought as I walked away, and then I started to think about it. How often do you actually have a "prescripted conversation" with someone that you know or don't know? I bet you do it all the time, although if you don't reply to someone that starts one of these "conversations" you are considered rude, stuck up or just a dick. What if you really just don't have anything to say or you don't want to say anything because that person probably doesn't even know you? Why is it that most people seem to think that it is necessary to actually say something to everyone they walk by? These people should know that not everyone wants to speak to them, yet they insist on speaking. I know that it is considered common courtesy to actually acknowledge someone's existence, and that it is customary to give the greeting of the day to those that out rank you in the military. It's also considered just polite to say something but why do we all use the same form of greeting? For example everyone has heard, "what's up?" "How ya doin?" "Good morning" "Afternoon". When you actually have a conversation with someone do you start with these or do you actually have something interesting to say? I'm not saying I don't use these most of the time, I just wonder why?