Human behaviour mind boggling I tell you

I love sitting and watching human behavioral patterns. It is so fascinating.
On a daily basis we work with our immediate team and others within the building that our sales team is based.
Every morning when my immediate team arrive we all greet each other and ask about the morning so far or how their evening was etc however it baffles me that other team members that arrive who share the same floor with us literally a meter away from our desks do not greet on arrival and hours can go by until you interact over picking up something at the printer and they start engaging in conversation with you for the first time for that day as if we had chatted earlier and there is no acknowledgement that this is the first interaction for the day. Some others come into the office and start chatting about work without a "Hello" and they go on as if they have seen you several times for the day. Do people just take each other for granted that they will be there on a daily basis and there is no need for greeting??Hang on.....I feel the same when collegues sitting within meters of you get up and leave for the day without saying goodbye. Initially I use to find this outright rude and then I realised that they are "sleep walkers."They are just not in tune with the moment and are already thinking of a million other things instead of being in tune with their colleagues and surroundings.
I actually pointed it out to a colleague the other day that I greet him every day and that sometimes at 1pm in the afternoon we will have our first bit of banter for the first time even though we can see each other from where we sit. He greeted me first for the first time the other morning and we had a good chuckle however nothing has changed.
We also attend management meetings where we interact with other managers that we hardly see in days and when everyone arrives in the boardroom people just sit down and don't greet?????
Human behavior-mind boggling I tell you!!!!!