well in the series theres many freebie power ups.
Gohan getting that mystic gohan thing, Goku becomeing a super saiyan god, goku drinking the ultra divine water, goku pulling a SSJ3 transformation out of his butt.. and then theres the zenkai boosts too of course.
Heck, those zenkai boosts are one of the biggest flaws in the entire series. After the frieza saga they dont appear, and considering how big these boosts are its weird how low the power levels of other saiyans where considering they fight all the time.
Also makes you wonder why they never got the idea to grab a bag of sensu beans and two saiyans just taking turns shooting each other to near death, eat a sensu bean and repeat that again untill they are like gods. Or get a healer like Dende to help, even easier since sensu beans seem rare because Jajirobe ate them all in dragonball.