Why did Pettigrew wait 13 years to return to Voldemort? Couldn't he have just magicked a rat to life, like you do birds with the Avis spell (Bird-Conjuring Charm), and put it under Imperio to copy his rat-Animagus lifestyle, in his place to pretend to be Ron's pet, while he came back to Voldemort?
Pettigrew was a coward who wasn’t loyal to the Dark Lord.
Pettigrew wasn’t serving the Dark Lord out of any true loyalty to him - he was only doing it for protection and to ensure himself a more comfortable life. He wasn’t going to simply choose to seek out the Dark Lord or continue being loyal to him when the Dark Lord seemed to be either dead or powerless - there was nothing in it for him then.
“There!’ said Pettigrew shrilly, pointing at Hermione with his maimed hand. ‘Thank you! You see, Remus? I have never hurt a hair of Harry’s head! Why should I?’
‘I’ll tell you why,’ said Black. ‘Because you never did anything for anyone unless you could see what was in it for you. Voldemort’s been in hiding for twelve years, they say he’s half-dead. You weren’t about to commit murder right under Albus Dumbledore’s nose, for a wreck of a wizard who’d lost all his power, were you? You’d want to be quite sure he was the biggest bully in the playground before you went back to him, wouldn’t you? Why else did you find a wizard family to take you in? Keeping an ear out for news, weren’t you, Peter? Just in case your old protector regained strength, and it was safe to rejoin him …’
Pettigrew opened his mouth and closed it several times. He seemed to have lost the ability to talk.”
- Harry Potter and the Prisoner of Azkaban, Chapter 19 (The Servant of Lord Voldemort)
Even after he rejoined the Dark Lord, it was only because he’d been revealed as a traitor and unregistered Animagus, meaning he’d have to go into hiding and couldn’t continue to live as a pet.
“My Lord! I – I have no wish to leave you, none at all –’
‘Do not lie to me!’ hissed the second voice. ‘I can always tell, Wormtail! You are regretting that you ever returned to me. I revolt you. I see you flinch when you look at me, feel you shudder when you touch me …’ ‘No! My devotion to your Lordship –’
‘Your devotion is nothing more than cowardice. You would not be here if you had anywhere else to go. How am I to survive without you, when I need feeding every few hours? Who is to milk Nagini?”
- Harry Potter and the Goblet of Fire, Chapter 1 (The Riddle House)
The Dark Lord knew Pettigrew regretted returning to him, and only had returned to serve him when Pettigrew had no other options. He’d only rejoined the Dark Lord when he was driven out of hiding.
“And then, not even a year ago, when I had almost abandoned hope, it happened at last … a servant returned to me: Wormtail here, who had faked his own death to escape justice, was driven out of hiding by those he had once counted friends, and decided to return to his master.”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
He didn’t attempt to rejoin the Dark Lord before then because he didn’t really want to.
Sirius Black puts it perfectly in "The Prisoner of Askaban", when he accuses Peter Pettigrew: "You'd want to be quite sure he [Voldemort] was the biggest bully in the playground before you went back to him, wouldn't you?".
After Voldemorts demise Peter Pettigrew needed to hide from the Deatheaters, who thought he was a double-traitor, as well as from all other wizards, who thought he was killed by Sirius Black (and only thought him innocent because of that). In short, until driven out of his hiding place by Sirius Black and Remus Lupin, Peter Pettigrew had no incentive to go to his now powerless or dead master.