Have DBZ fans ever asked the question just why is Goku stronger than Vegeta?
Just think about it, Vegeta is at least shown to train harder and more often than Goku soy why can’t Vegeta ever surpass that so-called “orange wearing idiot.” Is Goku just a naturally gifted warrior similar in genetic genius to the likes of Neji Hyuga and Sasuke Uchiha?
Nope, if anything Goku is best described as a Genius of Hard Work!
Saiyans had their power levels tested at birth. If they were weak, they were put in a spaceship and sent to go conquer planets as children. This, in fact, happened to Goku, who was born horrifically weak by Saiyan standards.
Vegeta, on the other hand, was born an Elite. A badge he wears with excessive pride. At no point in his life before coming to Earth did he have to struggle for his strength - it was just there. If he needed more, he trained, and he achieved it.
But that's not how things went for Goku. Goku didn't just train, Goku learned. He studied a formal martial art from a very young age, and from there was taught by anyone who would teach him and learned from everyone who fought him.
Goku had to constantly look for and learn new ways to get better because just sparring until he was the strongest wasn't a thing he could do. And along the way, he became the best at learning, at least as it applied to fighting.
He could watch someone do something once and immediately mimic it - even if it's something he had no idea existed before, like when he learned his famous move the Kamehameha. Then, he could apply it in new ways that the creator had never before considered. At one point even firing it out of his feet.
This doesn't just apply to techniques, though. Goku learned how to train in a way that Vegeta simply never did. Vegeta was significantly stronger than him on Earth to the extent that Goku had to use a technique to multiply his power level several times over just to fight him as an equal.
When Goku reached Namek, even though Vegeta had been getting exponentially stronger the entire time he was there, he vastly surpassed him. Why? Because years before, when he was a kid training with Master Roshi, he learned the advantages of training with higher weight and took that to its natural conclusion with artificial gravity training on the way to Namek.
Goku and Vegeta both spent the same amount of time in the Time Chamber before Cell and, once again, Goku trained smarter rather than harder. He realized the limitations of the Super Saiyan transformation and rather than trying to give it more raw power like Vegeta did, he explored the form and mastered it until he had eliminated its largest weaknesses.
Pretty much every time Goku suddenly surpasses Vegeta, it's because he out-thought him. He spent years learning how to get stronger, and because of that, he can still do it more efficiently than pretty much anyone else.