> First data is readily available, so if they want to know they can.
Yes and no. Yes industry numbers are readily available but I have friends that I consider to be at or above my technical level who suffer from an almost crippling imposter syndrome or "Yeah, maybe in CA but I couldn't get that wage in KY". Also all the companies I choose to work for are smaller (<100 or even <30) and so there is much less data out there on sites like Glassdoor and/or there could be potential backlash for posting your salary since it would be trivial to find out who posted it (even with fudging title/salary). Yes, the backlash would be illegal but it can be extremely hard, if not impossible, to prove and so it's easily to wait till I've left a company for a while before posting that data publicly.
> Secondly I've found that in general people value familiarity and stability more than maximizing salary.
There are absolutely people I've shared info with that decided to stay but I'm much more familiar with "You are making how much? Crap, I need to find a new job". If you are happy where you are and don't want to change then that's fine, I just want people to know what they are worth. Some people get stuck at a place for 5-10+ years and think "Well I couldn't find this salary anywhere else" or "What if I can't learn a new system/language/etc" and in a lot of instance I know these people could do it, it's just fear of the unknown (I'm not going to pretend I don't suffer from this as well).