Interviewing on the phone isn't always easy. You have no way to express or see the other person's body language which is a major part of how people communicate. Any audio glitches can cause a miscommunication. Any pauses on phone calls are awkward, but yet in technical work you usually have to spend more time thinking than actually writing code, so a technical interview is kind of awkward. Phone calls are a good basic filtering mechanism, but not really a great system overall.

Plus, while many people in technology are very friendly and nice people, some folks have a bit of social anxiety or are shy or any number of things.