It's a very good question.
In my case, my brother and sister live 1,500km and 2,000km from my parents, respectively. My brother prefers to drive when he goes to see them so it takes him two days to do the trip. I can beat that pretty easily when flying from Taipei.
My Dad is one of those old blokes that prefers to think that he is independent and can pay his own way. Even if he needs to live a nursing home in the future, he'd prefer that to being a burden on his kids.
I agree with that 100%. Just knowing what they like is probably part of that. Your boyfriend probably knew that they like to hear his voice. My Mum prefers a hand-written letter to phone calls or emails. I try to write one every two months. It helps me feel a bit less guilty about living so far away.