The plot errors of dragonball they are just as impressive as Goku’s power in his Super Saiyan Blue state. It all goes back to a simple wish made in the first episodes of the manga, which has led to a lack of coherence in subsequent sagas, including Dragon Ball Z and Dragon Ball Super. Get ready for a trip back in time, when Goku was just a kid.
It was in the Red Patrol saga, specifically in chapter 111 of the dragonball original, in which Shen Long tells a young Indian boy named Upa that there is no wish he cannot grant. The eternal dragon makes this confession after hearing Upa’s wish to revive his father Bora, something that instantly comes true.
The problems happen in the later sagas of dragonball. The first time Shen Long was unable to fully grant a wish was after the Cell Games in Dragon Ball. The dragon god claimed that he could not revive Goku since the spheres had revived him before. We later learned that Shen Long was also unable to revert Androids 17 and 18 back to normal humans, as their power exceeded his own.
Upa makes her wish to Shen Long

if we check Dragon Ball Super, we will notice that things get even more complicated. Shenron was unable to summon a Super Saiyan God to fight Beerus as none existed at the time and Dende had to upgrade the spheres to allow Shen Long to grant the wish to unlock Piccolo’s potential like the Grand Patriarch did for Gohan. and Krillin, a fact that was learned in the movie Dragon Ball Super: Super Hero.
You also have to consider the limitations of other Eternal Dragons. Namek’s Porunga was unable to bring Goku’s soul to Earth’s registration station because Goku was not dead at the time. The case of Toronbo, the dragon god of the Cereal Planet, is special because he does fulfill wishes without warning of his side effects.
So why didn’t Shen Long tell Upa the whole truth if there are things he can’t do? Perhaps the dragon god deliberately lied to avoid fulfilling wishes that were too complex or put the recipient of the wish at risk. Another answer would be that Toriyama changes the rules of the game so that the drama is more complicated for the characters and thus energize the fights.
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