Well, I mean... If players are able to walk through each other, it would make sense for npcs to share the same properties. As unrealistic as it is in the first place, it kind of goes with. I could see not being able to walk through npcs annoying under certain circumstances, as well.
Then how come we aren't able to phase through walls and other objects?
The phasing skill only covers solid, living, and/or animate objects.
We can slightly vibrate our molecules to pass through other moving molecules, like other humans and other animals. But to pass through solid objects we'd have to figure out a way to completely morph our body to fit in the little space in between a solid object's molecular form.
Legend has there's a spell to do this, but it's locked away in the depths of the Wizard City library, never to be seen.