The reason is that Necromancer damage is mostly physical.
It's actually another skill tree that increases the damage. I wanted to make Necromancer, but later realized that Necromancer skills don't work as other 'magic' skills. I'm even not talking about 'min-maxing' perversion. You can respec, but there are buffs/debuffs from certain events that can't be removed. You are free to mix skills, but it's very easy to f-up and make an inefficient class/build. Class/skill system is mediocre at best.
