jfem,
I would be one of those people that wouldn’t greet a non-related woman on the street. However, if somebody would greet me, I would definitely reply which would after all be the polite, normal and Torah way.
I think why some men don’t reply is 1)They are not used to it and caught by surprise 2) Since it does not happen to them often instinctively “think” that the right thing to do is to not respond. If they would ask a Rabbi or have time to think about it most would probably agree that they should reply. 3) If somebody knows you personally and feel that you are trying to impose your outlook on life and your values on them, then they might not reply.
BTW don’t let strangers’ comments get to you. They don’t know you or your personality. Take it in stride.