When did it become acceptable for people to not return a phone call, email, text, etc.....

We live in a day and age where most people have smartphones and can respond in seconds. A quick look down and done. We have become such an "impersonable" society with advancements in technology. I realize that this may seem a bit calling the kettle black as I sit and type this on a computer, but think about it, we as a society can go an entire day and not interact with a single person. Everything is automated or "there's an app for that" these days. I will not even go off on Facebook, Instagram, and Twitter because I think that has been attacked by more people than I can imagine. As I explore new opportunities in life, I go through the processes of reaching out to people through the proper channels, proceed with conversations and then nothing. I'm in no way naive to the ways of the world, however where are the basics of common courtesy, follow up, or even a thanks but no thanks. Are these not the basics upon which a society is supposed to function? Are these not the ways in which exchanges are supposed to work? I find it appalling after being on both sides of the coin that little things that I will do to begin or end a conversation are overlooked by other "professionals" I've said it before that communication is the key to society, everyone interacts with someone, whether text, email, or voice, and to not respond is downright infantile. What causes one not to send a quick auto reply note or a template email?

When someone makes the conscious effort to make a phone call, send an email, send a text, why not respond?