Are you seriously connecting real life situations with manga and anime? If it's sarcasm, you're a genius, if it's not, then I stand with Blazing Barrager on this one.
As a Human in Dragon Ball, you simply possess Saiyan Blood, as little as it is, it's there. Never in Dragon Ball, even Dragon Ball GT, it was clearly stated that diluted Saiyan blood would make the Super Saiyan transformation impossible to achieve(if there is, then you all can punch me in the face). The Pan excuse is useless, we all saw Vegeta Jr and Goku Jr turn Super Saiyan without sweat after unlocking the transformation. As macdaddymario stated though, it's still a video game, which means they MUST put aside part of the lore to balance out things. So yeah, becoming stronger, it will always be about training and experience. The entire Dragon Ball series is mostly based on the fact that Goku has to train and get stronger to fight stronger enemies as the whole point to his character IS NOT being a freaking hero but finding the best opponent. That, unless an author takes advantage of the classic plot device of the "gifted" just like Toriyama did with Gohan and later with Goten and Trunks.
To me, it all depends on the race: an example would be the fact that in the game, Namekians at the same level as humans in their base form will always have higher power levels, pretty much just like Piccolo(before Super) didn't need any kind of transformation because, as a namekian, he could control his Ki much better than Humans and Saiyans in their base form.
You need to always keep in mind that Humans were never that strong to begin with(that's the whole point of Goku, a low class Saiyan, sent to Earth to destroy it), and neither were they able to control Ki before Master Mutaito founded his school. Where as Namekians and Saiyans had great Ki potential from the start, proven by the fact that Dende clearly states that they learn to fly since childhood. Majins? They are Buu's offsprings, of course they have great Ki potential.
That single fact should explain why the SSJ is precluded to Humans in DBO unless Shenron awakens the power inside of them, as Blazing Barrager said: "We are unlocking the hidden potential that still exists despite no longer possessing the power it was known for".
Well that's only half right in the lore. After the manga and Goku's death in Age 801 (most likely by old age since he and Vegeta left Earth to have one more fight and were never seen again), Gohan released a book called "Groundbreaking Science" that explained the fundamentals of ki to the Earth's population. I believe it wasn't that Humans could control ki to the same level as the other races, but simply because they were ignorant about it (Mr. Satan aka Hercule didn't make things better). Now granted they aren't as "good" with it as Saiyans and Namekians, but that doesn't mean they were weak. Tien is a prime example of this when he was using Shin Kienzon (Neo Tri-Beam) to stall Semi-Perfect Cell. Despite being heavily outclassed in power, Tien was still able to perform a feat on an opponent who was stronger than a normal, untrained Super Saiyan.
Now the issue with Humans in DBO is the fact that our CaCs are descendants of the Son/Brief family. As such, while our CaCs Saiyan blood is pretty much gone, the potential that goes with it remains and simply requires unlocking it. This was something that Dende was actually counting on when he reactivated the DBs and made more of them while placing huge restrictions on what the user can wish for.