But why doesn't jaeseokcheon know he created that crappy universe ┑(￣Д ￣)┍

He didn't. The developer did. He was overwritten. The act of pulling the developer inside the game was his last resort of saving the heavenly realm from being destroyed. If anything he's a real king and true ruler with great decisions. He didn't hand his powers out easily to the MC, he thought everything so well that he was counting on the fact the developer knew the game so well and he'll understand the messages behind the missions he gave the MC