A fair question. Necromancy is one of the forbidden schools of magic, but what makes them forbidden? We understand why mundane crimes are punished, like murder and theft, but what of magical crimes?
The answer is, in each case, the consequences. Lets take a look at one necromancy spell in particular, and break down how it functions. The ability to call the dead back to a facsimile of life.
See here, this part of the spell identifies the soul linked to the corpse. This part of the spell reaches out into the Great Beyond and finds them, this this invocation changes the seeking spell into an ensnarement and pulls the soul back into the body, at which point the final invocation triggers and the soul is melded back into its former place, animating the body. The spell is now over, and the corpse is possessed by the soul of its former occupant.
Do you see the problem? It's okay if you don't, a lot of people miss it at first. Lets give a practical example:
A man has been murdered, and no one knows who the murderer is. The village people come to you to find the answer, so you raise the corpse of the man to ask him who it was that killed him. He answers your question, and your business with him his now done.
What happens to the revenant? The answer is... nothing good. You have brought this man back from death, but he is trapped in a body that is no longer alive. A body that will continue to rot and deteriorate until he can no longer see, hear, speak or move. He cannot be killed a second time, because how do you kill a corpse? The presence of the soul in the body no longer depends on the body being FUNCTIONAL, burn it to ashes and the soul is still unable to pass on. The spell that brought life to the corpse is over, it can't be ended because it has ALREADY ended.
In bringing back this soul, you have unintentionally created for it a state where it can never know rest of peace. A fate worse than death.
We don't do forbidden spells for a reason.