Goku was much lower than a Saibaman when he died, while Yamcha and Tien were much higher than Saibamen.It was a completely stupid piece of filler they added to the anime that made no sense.
Goku trains with King Kai for 3 months and increases his power like 15-20x.
Tien, Yamcha, and Chiaotzu train for less than a week and they go from Saibaman-level power to Ginyu Force level. That's like, 25x the rate of progression of Goku.
Fuck, Tien took on both Jeice and Burter at the same time.
And Goku learned the Kaioken and the Spirit Bomb techniques from King Kai directly (after running long distance, and then learning to run in high gravity), while Yamcha and Tien appeared to have just spent the time sparring with each other (in high gravity). Chiaotzu doesn't matter because his opponent was a weakling whose only power was stopping time.
And DBZ is all about multiples. It doesn't matter if Goku got 20x benefit from Kai's training and Yamucha got 40x, because all it means is that Yamucha squeezed in one more 2x multiplier.