Let us reason from cause to effect:
- Man is unconscious in death. Eccl 9:5, 6, 10, Ps 6:5, Isa 38:18, Ps 88:10. Thus, the Bible very often calls death a “sleep”; Matt 9:24, Mark 5:39, Luke 8:52, John 11:11, 12, Acts 7:60, 13:36, 1 Cor 7:39, 11:30, 15:6, 18, 20, 51, 1 Thess 4:13-15, 5:10, 2 Peter 3:4, etc.
- The soul cannot think after death. Ps 115:17, Job 14:2, 21, Ps 146:3, 4 “their thoughts perish, KJV).
Therefore, any attempt to communicate with the dead is either an exercise in futility, or more importantly, an attempt to communicate with something that is not dead and impersonating the dead and thus deceiving the seeker.
Necromancy was a widely practiced among the pagans as a form of spiritism. Even if valid (which the above shows it was not) it is still forbidden because God wanted the Israelites to seek divine guidance from either God (via the prophets) or from the Scripture:
Isa 8:19, 20 - When men tell you to consult the spirits of the dead
and the spiritists who whisper and mutter, shouldn’t a people consult
their God instead? Why consult the dead on behalf of the living? To
the law and to the testimony! If they do not speak according to this
word, they have no light of dawn.
This ban of necromancy effectively limits God's people to a single channel of divine information with no competing sources of data. Put another way, the ban prevents someone from saying something like, "The Bible says this but when directly consulted my great-grandfather, he said this!" - or any other dead "authority".