If their physical, earthly bodies are dead, communication with them is forbidden.
Besides, people in hell are alive too, so that argument falls flat.
We're not conjuring them up, like the witch of Endor, or engaging in two-way conversation.
We're asking them to pray to God for us.
Rev. 5:8And when he had taken it, the four living creatures and the twenty-four elders fell down before the Lamb. Each one had a harp and they were holding golden bowls full of incense, which are the prayers of God's people.