Because, I don't know, IT'S NOT A PRAYER ?Really? Is this a prayer?
"Yea though I walk through the valley of death, I will fear no evil: for Thou art with me; Thy rod and Thy staff they comfort me."Well? Is it?
The passage from Psalm 23 is part of the book of Psalms, which are specifically songs written to praise God.
So they can't possibly be considered equivalent.
Second, Jesus, Himself, was asked how to pray, and responded with what we now know as the Lord's Prayer.
If we as Christians, were to pray to saints or others such as Mary, why didn't Jesus, Himself, mention Moses or someone from the book of Judges? Or David, or Solomon?