There is no indication in the Bible that people can talk to the dead or that the dead can hear what we are saying.

When we talk to a grave, we are saying that they are in the grave, six feet under ground.  They are not.  Their bodies are there but not their spirits.  Their spirits are with God...or not with God, depending on their love and acceptance of Jesus Christ.

While they are in that spiritual place, why would they want to hear from us?  If they are with God (or with Abraham as indicated in Luke 16), they are in a much better place.

Hopefully they are listening to God and praising him.  Their minds are consumed with God, his beauty and presence.  The problems of this earth seem petty in retrospect.

Those who are not with God would long to hear from us, but part of their punishment is separation.  Loneliness is the sentence.  If hell were a party, it wouldn't be hell.  Hell is being alone, away from anyone who cares.  For that reason, it doesn't seem logical that God would allow them access to the world.  They chose separation and separation is what they are experiencing.

While it may provide comfort for some to talk to a grave, the real comfort is knowing that they are with God and he's taking care of them.  Instead of talking to a grave, talk to God and establish a relationship with him.