Q: Why does the priest celebrate with his back towards the people?
A: He doesn’t! He celebrates facing the Lord WITH the people.
There are two visuals that guide our orientation in worship. First is to visualize the altar as the Lord’s Throne and the tabernacle, which contains consecrated bread and wine, as His Real Presence. Thus when the celebrant prays with and on behalf of the congregation he should be facing the Lord with the people.
The second visual is to think of the altar as the end of the heavenly banquet table breaking into our time/space world. On the other side are the angels and archangels and all of the company of heaven i.e. the communion of the saints. If the altar is off the wall and the priest stands on the other side, facing the people, this important symbol is lost.
Lastly the priest faces the same direction as the people because he is not the host of this sacred meal. Christ is the Host and so with the priest not facing the people it is easier to look past the priest to our Host.