Because they do nothing while "in hiding". Giving an explanation for that doesn't really change it. So, effectively, they only exist and act one at a time and Wizard is always able to track them as soon as they become active. It means that nothing will ever happen outside of the immediate reach of the hero.
I agree. W, OOO, and Fourze all gave us in-story reasons they villains couldn't simply overwhelm the hero while he was still weak. In Wizard, as far as they have shown, the episodic structure seems counterproductive (in-story). Would a massive attack of monsters that sends the entire city into chaos make it easier to get people to fall into despair?
One reason why I can think Wiseman keeps the Phantoms in hiding would be that he doesn't want to trigger a direct confrontation with the White Wizard and/or whoever else might be part of that universe.