Why? Because Christians essentially do not own their bodies.
God Owns You
You shall not make any cuttings in your flesh for the dead, nor tattoo any marks on you: I am the Lord. Leviticus 19:28
If anyone defiles the temple of God, God will destroy him. For the temple of God is holy, which temple you are. 1 Corinthians 3:17
Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; you were bought at a price. Therefore honor God with your bodies. 1 Corinthians 6:19-20