Why So Many Preformulated Prayers?

Because "we do not know how to pray as we ought" (Romans 8:26).

In fact, most Orthodox prayers come from or are rooted in the liturgical services of the Church, and are of ancient origin.

Also, I've found that using Orthodox prayers helps me become always 'more Orthodox,' helps me grow morally and spiritually. By the Graciousness of God.

So, pray away!

BTW, here's another common source for Orthodox prayers.