Wouldn't you agree that most regular pencils have better interfaces than most regular pens? I see at least two reasons:
1. Troubleshooting: If a pen stops running, it will take you a while to figure out why. If pencil stops running, you know it's because the tip is either broken or the graphite is too short. Both problems require only one solution.
2. Status indication: You always know how much of a pencil you have left just by looking at it.
Text via: Advertising Lab, Image via: Marrael