A good question.... But it could be extended to banks and post offices, too! What on earth is someone doing at the teller window at Lloyds TSB for 20 minutes?! I mean, seriously, what?

And the post office. Weigh it. Pay it. Post it. Few minutes.

People insist on making idle chit chat with the staff (when the staff are also obviously not interested) while a queue of others wait behind them "patiently". We're so British.