"The priest is not talking to you"
Protip: Follow along in your missal, 'specially the part where it says "Secret"... :D
The Priest leads us to God and we follow his back. He leads us on a Heavenly pilgrimage to the Heavenly Jerusalem (East or orientation). A bus driver leads us to an earthly destination and we take comfort in watching his back and the drivers tentativeness to the road. What if the bus driver was to turn around and face the passengers?