What's up with people that don't smile ?

So I want to ask why is it when people lock eyes with you they just stare at you emotionless? It happens to me everyday, 90 percent of people if not more will just look at me emotionless even though I smile at them.. because like that's fucking normal ? I find it a bit rude when people just stare at me and don't smile back ? Sure you don't have to say hi..

My question is why are people like this? And does anyone see a foreseeable future where people change this habit ?