As I said in another thread:
I think Doom can't recreate the Universe because, despite all his blunder, he's psychologically crippled.
He has gotten absolute power several times before, and he always messed it. During the previous Secret Wars he basically defeated himself because he was unable to control infinite power, and when he got the Infinity Gems he sabotaged himself by creating the beings that would take them from him. Another time he got Galactus's power AND a Cosmic Cube and he forgot to watch over the Cosmic Cube. And yet another time he got the power of the Lifesforce and he almost killed himself by absorbing too much of it...
I think self-doubt has finally creeped into Doom's soul. He must be terribly afraid of repeating his past mistakes, he probably keeps watching himself in search of flaws or weaknesses, the chink in the armor that will make him loss his power again.
In addition to that, he recently has been forced to face his imperfections: His defeat at Scott Lang's hands, who could see through him and told him to his face his true nature, his exposure to the power of Loki's Sword of Truth, his time of introversion while inverted during Axis. Deep down he knows that he's unworthy of being God, and that gnaws at his soul. He lives in fear of the moment in which he will fall from his throne yet again.
Plus he knows that he has stolen Reed's family. He knows that he deserves punishment, and subconsciously he expects the man who has so many times been his undoing to come to get him yet again.
Hence the scarred face: It's the expression of his self-doubt and feelings of inadequacy.
Hence the lack of Omniscence: He's afraid of the sensory overload, of the distractions that will make him miss the dagger in the back.
Hence the lack of commitment to the rule of Battleworld, and why he doesn't dares to try to restore the Universe: He's afraid of using too much power.