Because we all have one and we all tend to communicate with it than per person or by confidence.
Last year, when I was in construction field, there was this contractor who like to play hide and seek with me, mobile phone is the only way to communicate and if one is not picking up or reply back, basically that person must have been missing on this World. Imagine how nervous your parent/you will get if you/your child don’t reply your call or smses.
The invention of mobile phone helps we, become not only lazy, but less responsible at the same time.
You don’t pick up calls you think will get you in trouble; you don’t pick up when you know you have RV loans to pay; you don’t pick up the call when you know you will be scolded for something you’ve done; you don’t pick up the call when you know it will annoy you. I can go on and on for the list.
It is easy to just press a button, but it will never solve your problem with the ringtone.
I like all Apple product design, but not necessary the product features itself.