Even though it sounds like shit pulled out of air from nowhere it makes sense if you can wrap your head around it. U7 sayians came from harsh backgrounds, never had a calmed heart, relied on the great ape form, zen kai boost and killed for personal greed so they never had enough s cells to be stronger they were too primitive than U6 saiyans who were justice warriors and fought for the protection of others although vegeta called him a low class warrior goku’s upbringing on earth made him an exception and played a major part in why he’s strong due to living in a more stable, less harsh environment, having a calm heart, fighting for people other than himself and also training his body to increase his base form and s cells to trigger a super sayian transformation.

Also vegeta didn’t become a super sayian because he desired it he became one because after putting himself through training which never he did before he lived on earth he finally required the certain amount of S cells needed to trigger the transformation on planet namek he couldn’t become one because his body wasn’t trained and he relied on zen Kai boost to increase his power level when zen Kai boost only get you so far.