Why don't some people respond to emails/texts?

I find this to be really rude and a shows a lack of courtesy/manners.

And no they are not ignoring me.. because occasionally they would send me a random email or call me up to ask me how I am.

For those of you who do this or have had this experience, can you give me some answer as to why some people are like this ?

What particularly annoys me is that these same people expect you to answer right away when they send you an email or text.