Because Jesus wanted to show the tax-collectors that all thing are from God.

Jesus' penalty for "sin" needed to include physical and emotional pain because God said that the punishment for sin is death.  A person is made up of physical, emotional, and spiritual aspects.  For the complete punishment of sin to occur, it must include physical and emotional pain.  One experience in one of the parts of man, either spiritual, emotional, or physical, affects the others.  For example, when we are emotional bruised, we feel it physical (as in tears or stomache pain) and there is spirtual pain that occurs, because there may be a disconnect of your spirit with the one or thing that hurt you.

Spirtual separation alone is not sufficient penalty for sin because sin is very ugly and deserves the worst punishment, demonstrated by how Jesus was treated and how He died on the cross.