Cast on Skills and Raise Zombie/Unearth

Why can't we socket Raise Zombie or Unearth into the "Cast On" skills? Raise Zombie is mostly useless outside of some pretty specific circumstances, but if I could set it up to work automatically as part of a "Cast On" skill it might be worth it.

In POE1 I had this cool enchant that would "Use socketed spell gem if another skill is used" and I put raise zombie in it. So any time I would cast any spell it would raise zombies at the location.