If your entire extended family is iPhone only (which mine finally is since my sister and her husband finally converted from Android) Facetime becomes more popular than regular phone calls. For us, it is simply just "the way" you call.
I know we can't be the only ones...
BTW: You are right about Apple originally claiming that Facetime was going to be an "open standard" and they were going to allow other clients to connect to it. That's all gone out the window though as Facetime has become a differentiating feature vs. Android.