Because praying to the dead is a sin?
Praying to the dead is strictly forbidden in the Bible. Deuteronomy 18:11 tells us that anyone “consults with the dead” is “detestable to the Lord”. The story of Saul consulting a medium to bring the spirit of the deceased Samuel back to life led to his death “because he had been unfaithful to the LORD; he did not keep the word of the LORD and even consulted a medium to be guided” (1 Samuele 28:1-25; 1 Chronicles 10:13-14). Clearly, God has declared that such things are not to be done.
God is omnipotent, omnipresent and omniscient
Consider the characteristics of God. God is ubiquitous, everywhere at the same time, and is able to hear every prayer in the world (Salmo 139:7-12). A human being, instead, does not have this attribute. Furthermore, God is the only one with the power to answer prayer. God is almighty (Apocalypse 19:6). Certainly this is an attribute that a human being, alive or dead, does not possess. finally, God is onniscient: he knows everything ( Salmo 147:4-5 ). Even before we pray, God knows our true needs and knows them better than we do. Not only does he know our needs, but he answers our prayers according to his perfect will.
Then, so that a dead person can receive prayers, the deceased individual must hear the prayer, possess the power to grant it and know how to respond in the best way for the individual who prays. Only God hears and answers prayer because of its perfect essence and because of what some theologians call its “immanence”. Immanence is the quality of God which causes him to be directly involved in the affairs of mankind (1 Timoteo 6:14-15); this includes answering prayer.
Even after a person dies, God is still involved with that person and their destination. Jews 9:27 says so:
“… Man is destined to die only once, and then to face judgment”.
If a person dies in Christ, goes to heaven to be present with the Lord (2 Corinthians 5:1-9, especially the verse 8); if a person dies in his sin, go to hell, and in the end all the inhabitants of hell will be cast into the lake of fire ( Apocalypse 20:14-15 ).
God provided His Son, Jesus Christ, to be the mediator between man and God (1 Timoteo 2:5). With Jesus Christ as our mediator, we can pass through Jesus to God.
Why would we want to go through a dead, sinful individual, especially when doing so risks the wrath of God?
