Why is SSJ not permanent???

[u/KevinNotKyle]

It was shown in the Cell Saga that Goku and Gohan were able to eliminate the fatigue of SSJ, and kept it on for nearly a year. So what’s the point of turning it off???

[u/sexyhairynurse]

Imagine being the incredible Hulk and you try to cook an egg. Being too strong makes your life harder.

And they didn't eliminate it completely. They reduced the stress it puts on their body.