Since the newly opened world offers such benefits, have there been any powerful beings who specially created new worlds just to cultivate their disciples?
Indeed, there have been, but that does not mean it can really be implemented.
Although Taoist Monarchs can indeed open worlds for nurturing many True Immortals and Supreme Strong ones, to comprehend the ways of the world, let us not even mention the great toll that such an act would take on the Taoist Monarchs themselves. Even if they were to open a world, the way of that world mostly belongs to the Taoist Monarch's own way, which is not suitable for everyone.
However, the Longevity Taoist Monarch is different. The breadth of his knowledge is evident from the teachings at the Taoist Academy; he seems to know everything and understand all sorts of systems and the Cultivation path of this universe.
Therefore, the world opened up by this Longevity Taoist Monarch is definitely suitable for the majority of them.
