I think some people are just rude and clueless, but for others I think it's a power thing.
I used to have this friend who would always take personal calls whenever we would hang out. I have no issue with people texting around me, or taking a call if it's important...but she would get on the phone just to chat. She'd do it when we'd be out at a restaurant eating or whatever, and it was so awkward b/c I would just be sitting there while she was talking. She would always glance at me repeatedly while she was on the phone with this weird self-satisfied expression, and then after her conversation was over she'd make comments about what she was talking about as if she expected me to have been eavesdropping...it was very strange, and I've never known anyone else to act that way. I think maybe it gave her some sort of sense of importance/power, since she basically put me in a situation where I had to wait for her to finish, with no one else to talk to, while she participated in an outside conversation that didn't include me.
The future is here, and it's about a hundred feet above the Arby's.