I think you also have to take into account certain medical issues.

My duaghter woudn't look people in the eye because she is Autistic. Very often, she will not respond to voice becuase if she is in the middle of doing something, she can only deal with that one task. Effectively it is difficult for her to multi task.

Having said that, if I greet somebody verbally, I expect a response. If not I assume they either don't want to talk to me or are being rude.

I generally say hello to people I am greeting and smile and offer my hand. I cannot remember a time when that hasn't been acknowledged, with the same response.

I guess it depends on how you deal with it yourself.