I don't get it... (warning-"downer" inside)

In the world of high school, when an adult says, "Have a great day!" is it totally uncool to reply "Thanks! You have a good day, too," or even just "Thanks!"? Do we no longer train our kids in the little pleasantries of life?

I have never been around people who ignore that sort of sentiment. Every single day I wish my students a good day as they are leaving. I thank them for working hard, for coming to class, for their "business." What do I hear in return? Nothing. I feel like it is so devaluing to be ignored, and I don't treat my students like that. I don't treat anyone like that, and I am ashamed of people who do.

I feel like it's second nature to be kind to people, but I must be wrong.