What's to stop a wizard doing this? Hmm...
I think you underestimate the time, energy, and expense in money and resources required to establish a safe working environment.
Let us imagine a wizard considering this idea. First, the has to find an appropriate plane. If he doesn'tknow of one, he has to experiment. Do you want to try random forks in Plane Shifting?
The first thing that pops into mind, when looking for temporally variant planes at random - what happens if you run into a plane that runs much, much slower than the one you start on? You leave, and then come back to find a century has passed? That's a big risk to run. Not to mention the risks of finding planes you cannot leave, that are hostile to life, etc.
Let's say he finds the plane described in a book, and the fork needed for Plane Shift. He can move up to 8 people to a random point on the plane. He won't return to that same point next time. Every time he travels, he needs to Plane Shift, then Teleport everyone. Now, under these conditions, he must find or create a safe place to set up shop on an alien world?
The thing that stops them is simple - It's a pain in the neck to get started. It's a heavy risk for little gain.