They wait on more than one person at a time. The line prevents them from being able to do that their way. A bartender might get 3 orders and start the one that makes the most sense to start first, do something else while the head is going down on a beer etc. While waiting for you to gather up your wallet and drink and get out of the way they're talking to 3 more people to your left. They don't go from order to change one person at a time while they bartend. It would take much longer.