I wave or nod depending if my hands or general attention is free.
I made a video that ends with me talking about it because as I was recording I nodded to another biker who did not nod or wave back. The video is called "Stabilizing Rear Brake & Why I Wave" or something like that.
The short answer, without forcing you to watching the video, is that I wave or nod for myself.
I want to say "hey, hi, hello. Ride safe and keep the rubber side down." or something. If they do not wave back, that's on them.
I just assume they didn't see me or did not want to wave. So what? A judgment or not, it's their expression, not a fact or anything to do with me. That's fine.
I was not obligated to wave and they are not obligated to wave either. I wave because I want to, not because I expect anything in return.
I try not to take the actions or lack of actions of others personally. Putting one's happiness in the hands of others is foolish.
They are living their life; I'm living mine. It's all good.