In the vast majority of the Dragon Ball universe the Saiyan are alien warriors who transform into the Oozaro under the power of the moon, but in this timeline Saiyans are monkeys 100% of the time! So, when Goku the legendary warrior appears on your front door asking for your assistance in tracking down the Dragon Balls how could you resist? (In this timeline Chi-Chi and Goku aren't together at the moment, plus pre-Saiyan saga)