Another Zombie Problem

As long as I can't see two zombies on my screen, I can move around and summon more. Meaning, I could leave two zombies fighting one mob, run to another further down the map until I can't see them anymore, and summon another two. At one point, I had 20 zombies loitering around and I'm sure I could've summoned more if I wanted to.

E: and irrelevant to this particular thread, but portals don't show up on the map. They do in town of course, but not where they're cast. This won't matter in most cases given that we'll normally go back in through said portal, but if we return to the area through a waypoint, it'd be convenient to see it. But besides all that, it's really just because: why not?