Beacause his world would have had spiritanyway, as it was claimed to to be created in his image, and he himself admitted that he had spirit, and was thus incomplee.
While he might have created a new world, he would havestill not achieved his goal of a new world without spirit.
Cyrus never said that he was going to recreate a world "in his image," just that he was going to recreate it without spirit. A review of his quotes from Platinum on Bulbapedia reveals no instance of any version of the phrase "in my image." However, he did say that he intended to create a new world solely for himself, which implies that there would be no other living beings in his world. This is also evidenced by that fact that he regards the old state of the universe as ideal, the state in which only time flowed and space expanded. The state in which there was no life or spirit present to give rise to strife and corruption. A universe in which only he lived would be his ideal world, because it would only have his spirit to contend with, and he had proven that, except in the event of a mental breakdown, he was very apt at suppressing his own spirit. But even then, if he had the power to destroy every trace of spirit, then who's to say that he would be incapable of destroying his own?
Listen, you gotta remember these things; always avoid claims of Fennekin being (or becoming) part Fighting-type, don't tick off mods, and NEVER (and I mean NEVER!) suggest that Arceus is the God of Pokemon when Endolise is online. If you somehow make this mistake, run for the hills before he attempts to murder you via tongue lashing.