why do most people dont bother to say hello if they know and straight out ignore you?

I am not talking about the cashier or someone random on the street, I am talking about classmates, neihgbors and people you see on a regular basis or were introduced to at some point. I want nothing from those people nor am I trying to be their friend. But why do most people not engage in basic politeness like a simple hello? When is someone not worth basic politeness to you? I am not talking about someone on the street who may scare you or someone you had an altercation with, I am talking more about an acquantaince that you had no kind of conflict with.