Because in person you have body language and expressions to read as well, so you don't need to think about it as much. On the phone your brain has to work out extra detail that it wouldn't normally need to bother with.
Sometimes I can understand it's necessary to have urgent contact available. GPs on-call, fair enough. If you or your partner is expecting a baby, I'll accept needing the phone around. But there's just no need for it most of the time.
These people who you see getting onto buses talking and they talk all the time they're on and they get off still talking - what did they do before they had a mobile phone? Surely they didn't shut the hell up?