Mostly because necromancy in its core defies the cycle of life. Humans believe that everything have it's beginning and it's end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.  
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse. 

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes. Those people are evil.