It was so dangerous, and they could not bring back any treasures from the world. Such an illusion-type world was completely worthless to marauders. If slave vendors or interstellar pirates found such a world, they would wish to cut off their own hands and dispel this bad luck. Only a place like the Heavenly Gates would list a place like this to train their new talents and maintain their status.
"There are many demons in illusion-type worlds. Furthermore, various frightening things emerge from there in an endless stream. All kinds of things exist there beyond your imagination… To be honest, Boss, you are not from the Combat Cultivation Hall anymore, and they are not forcing you to go. Whoever likes such a broken place can go, but there's no need for you to risk it."