It wastes time if they're doing something more important and can't talk right now. Not everything I want to talk about is of earth shattering importance, so it isn't always worth their time if their doing something that is more important. If their time is more valuable sometimes, why is it bad that my conversation or question is less valuable than that? Sometimes I just want to catch up.
You will feel terrible if you don't pick up the phone and it turns out to be something important, like being able to hear the last words of your grandma or something.
Texting is a lot less of a big deal than a phone call is.