Same reason people at home just come up to each other and start talking (which actually requires immediate response) even when the topic is non-urgent whatsoever, instead of leaving notes around the house.

It's all based on differing conventions among people, so saying a call "demands immediate response" is putting your convention above others as the only true one.

In my family the convention is a bit different. A single call does not signal any urgency and so no one is expected nor obliged to answer if they don't feel like it. A second call after the first one wasn't answered implies importance. Third and more calls imply urgency and then emergency. If something is important or urgent and calls aren't getting answered, a message is sent.

I like my convention. I also have slightly different conventions with some friends. I am also aware different people may have different conventions and I don't hold mine to be superior or theirs inferior.