As someone said, you don't necessarily nod and greet every single person you see as you walk down the sidewalk, because if it's at all busy you'd be nodding constantly. Living mostly in busy towns and cities this is certainly true for me, and this may very well inform some peoples' behavior on their bikes.

For me, it depends. I generally say hello or something if I'm passing another cyclist, after announcing my intentions beforehand. I usually nod at oncoming riders but as others have said who knows if they notice it or not. Surely I've missed such things if I'm not looking directly at the oncoming biker. I've noticed that people will respond if you make eye contact, but again if your closing speed is pretty high that might not happen either. I don't commute so I don't see that many other riders regularly (there are one or two, as well as walkers, because I have a couple of routes I like to ride).