Because when I have 5 minutes before getting to work, I see a window of conversational opportunity.
But after the greeting and the customary back-and-forth "how are you?", I'm out of time. And most people think it's weird when I call without being able to dedicate my near future to the conversation, and ask to exit within 5 minutes of calling.
That's why my friend Justin and I have developed the modern-day phone conversation. It's continual. It never ends. It's 3 minutes one day. 6 the next.
It ends when one person says "Gotta jump." The other replies, "Later." We jump back into our lives. And we pick up the conversation the next day, right where we left off.
We have killed the formality, and because of that, communicate regularly.