You say it’s unbiblical, I say you just interpret the verses we cite differently.
Romans Ch 12
5 We, though many, are one body in Christ and individually parts of one another.
I believe this indicates all Christians are connected. When you die and go to heaven l, you are still part of this mystical body with other Christians. If I can ask a Christian here to pray for me, then I should be able to do the same with our brothers and sisters in heaven. There a lot more to this doctrine but that is the starting point of understanding it.