Well, because it was and is a requirement even of human nature. It is a requirement of prayer itself. When we pray, we address God. He is our goal. And since we are not angels, we have a body, so this interior orientation must be expressed bodily. We must be focused on the Lord.
When we are Catholics, we have an international structure, so we must have a concrete and visible sign toward which we are looking during prayer. Since the first century, this has been the holy cross of our Lord, the cross of our salvation, toward which we are all looking together in the same direction. This is the requirement of prayer itself.
You see the same in all other religions, in the mosques and among Buddhists. When they are praying, they are facing the same direction. No one is facing one another. This was an invention of the Calvinists, who transformed worship simply into a teaching lesson, modeling sitting at a table in a very anthropocentric form, because they had no sense of liturgy. The Calvinists in this form of Protestantism influenced our liturgy in the so-called toward-the-people celebration.
Another very important aspect is that the Holy Mass is substantially and primarily the sacrifice of the cross in its sacramental form. To offer a sacrifice, it is required that the priest, in the name of the people, stand before them and look with the people toward the sign of God, which is the Holy Cross, to offer to the Lord at the altar, toward the altar and toward the cross, the sacrifice of Golgotha.
Therefore, these requirements are so strong that the Church cannot continue to celebrate the most solemn prayer of adoration, which is the Holy Mass and the sacrifice of Golgotha, in such an anthropocentric manner that contradicts essential requirements of prayer itself.