Because a "spirit medium" is defined as someone who claims to be able to communicate with the spirits of dead humans.
Interesting, but where did you get this definition? Dead humans are not spirits. They are dead. In the Bible, which JWs claim to believe in, a spirit is clearly ALIVE, and is a demon, or fallen angel.
In the Bible, there were many approved spirit mediums. They were called prophets. A prophet is an approved communication with the spirit realm, and IS BACKED UP BY WORKS.
The GB losers claim to be prophets, but since their claim is false, they are reduced to the ranks of a crystal ball-gazing "spirit medium".