Because the writer says it is

In real world history, necromancy was limited to speaking with the dead and Christianity claimed that mediums were actually speaking to demons. No magicians ever claimed to animate corpses for labor until pulp writers invented the concept in the 1930s. Before then, the dead came back to life purely under their own power as ghosts, vampires, ghouls, werewolves and other man-eating monsters.

For an example of how arbitrary the morality is, look no further than the Necroscope series. The protagonist is a medium or "necroscope" who speaks with the dead and may call upon them to rise from their graves and assist him. The antagonist includes "necromancers" who mutilate corpses to learn their knowledge, which the dead souls feel despite being dead. Otherwise the dead don't care about the condition of their unfeeling remains.

Because the writer says it is, in real life it varied

• In some cultures, and even today, dead relatives are dug up and taken to festivals because the corpse was believe to contain the soul.
• In ancient Egypt, living relatives regularly brought offerings to graves in order to nourish the soul of the deceased.
• In European history, necromancy was limited to speaking with the dead and Christianity claimed that mediums were actually speaking to demons.
• The dead generally came back to life under their own power as ghosts or revenants. Sometimes, they came back as man-eating monsters particularly if the now soulless corpse was possessed by a demon.
• The concept of zombie slaves comes from the history of Afro-Caribbean religions and slavery. In the religion of vodun, a fate worse than dead is being enslaved after death. This is the horror of the zombie.
• In ancient China, urban myths circulated that sorcerers sold their services to reanimate corpses in order to transport them to receive funerary rites, in contrast to Haitian zombies.

If the soul isn't suffering, it's not wrong

If the soul is not suffering, then there is no moral reason that animation is wrong except for respecting property rights. The corpse, by most laws of inheritance, defaults to the ownership of surviving relatives who are free to do with the corpse as they wish. If they want to donate the body as a laborer in order to pay off bills rather than deal with the additional costs of a funereal, that is their right.

For an example of how arbitrary the morality is, look no further than the Necroscope series. The protagonist is a medium or "necroscope" who speaks with the dead and may call upon them to rise from their graves and assist him. The antagonist includes "necromancers" who mutilate corpses to learn their knowledge, which the dead souls feel despite being dead. Otherwise the dead don't care about the condition of their unfeeling remains.