In Dragon Ball, Goku is a unique saiyan warrior. He's not unique because he's pure-hearted (because guys like Tarble are similarly pure-hearted), but because he was born as a low-caste nobody but eventually ascended to surpass even saiyan royalty. Now, you could argue that he does this because he's totally committed to bettering himself. But that honestly doesn't hold up with what we see on-screen. Vegeta is absolutely obsessed with getting stronger than Goku, and despite his obsession, try as he might, he can't surpass Goku.

Today, I would like to offer a more detailed explanation. Goku is actually the product of a very unique set of circumstances that probably makes him a rather unique and unprecedented case among saiyans.

First of all, we know that Goku started off evil, but he turned good after hitting his head. But let's remember back to the scene where this backstory of Goku's was first revealed. It was when the Z-Fighters were meeting Raditz for the frist time. Raditz asked Goku if he hit his head as a kid. That was Raditz's first guess as to why Goku was friends with the earthlings he was supposed to destroy, and lo and behold, his first guess ended up being the correct one.

This suggests that this whole “infant concussion amnesia” thing is actually a well-documented phenomenon of saiyan biology. Why else would Raditz immediately turn to such a theory when he had literally nothing else to go on?

If this phenomenon is so well-documented, then that opens up the possibility that there might be other saiyans out there who hit their heads as babies and thus turned good.

So if there are so many others out there, then why haven't we heard about them? Why don't they ever join up with Goku and the other Z-Fighters in the everlasting war against evil?

My theory is that they never became as strong as Goku. See, if you really think about it, Dragon Ball's earth is kind of a weird anomoly, and not just because they have Dragon Balls. As we see in the frist 200 or so chapters of the manga, martial arts is a huge part of this planet's cultural identity. There are numerous martial arts tournaments (although the Strongest Under the Heavens is the most prominent), countless martial arts dojos, and several legends that survive on the wind's breath of mystical substances (such as Korin's sacred water or the ultra divine water) that can increase your strength artificially.

And yet, Dragon Ball's earth really has no business having such a huge martial arts subculture. They honestly aren't really cut out for fighting. After all, the whole reason Goku was chosen for the mission of destroying Earth despite being a baby at the time was because Earth had relatively weak inhabitants compared to the rest of the universe.

So this puts earth in a unique spot of having a lot of martial arts culture despite really sucking at martial arts. I would theorize that the majority of planets who have a strong martial arts culture would have similarly strong inhabitants. Thus, saiyans would send adults to conquer them. These adults would have physically matured to the point where they would no longer suffer amnesia if they were concussed. Meanwhile, babies – who are vulnerable to concussion amnesia – would be sent to weak planets who have pacifist cultures to match their relative strengths.

This is the main reason why these other saiyans are never heard from. Because the planets they are sent to do not afford them many opportunities to increase their strength, when the adult warriors come to check on them, they may try to fight back but are completely ill-equipped to do so.

Earth, meanwhile, is the only planet that has yet to be marked by Frieza's forces for conquest that not only would call for an amnesia-susceptible saiyan to handle the mission but still provide that saiyan baby with the means of reaching their full potential after suffering said amnesia.

This would explain why Goku is so unique.

So what do you guys think?