Is it because when they are together they are doing other things also, but when they are on the phone it's purposeful and the have something they want to say?
I definitely see what you describe, specially with millennials and younger. So many who grew up with cell phones or computers can't converse well in person. I really noticed this in the past few years when I started making friends with people twice my age, they are so much more interesting and talk for hours.
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