The NPC city is hard to conquer because the NPCs have fortified their cities to have extremely strong defenses. Even the pope-level forces can't necessarily take them down with all their might.
Even when such forces unite and act together, they rarely gain any advantage over the NPCs. Because of the power of the cities and the advantages of mobilizing combat power, the NPCs have not been fully suppressed by the monsters, even if their strength is far inferior to the monsters.
However, apart from Zhou Tian's previous feat of destroying the NPC Novice City, relying on Zhou Tian's current strength, he certainly won't stop because of the convenience of the NPC cities.
