Why can't goku go super saiyan 5?

Goku did not reach the elemental emotional and physicall stress in order to become of such level "five". As far as I know saiyans/sayajins do not have limits. Goku and Vegeta both surpasses what they called "limits". Super saian, super saiyan 2, super saiyan 3, i am not sure about ssj4 because it was not cause primarily of stress. Also, for saiyans, anger is there power, the more angry you become, the more power you attain. Though, at the very stressful times, Goku and Vegeta did fusion (Vegito and Gogeta) to defeat their enemies other than getting so motherf***ing angry they would be filled of rage and kill anything. The point is, they did not feel fear, saiyans do not necissarily feel fear for a long time...they get so pissed they transform and kill ennemies. If their was something to push them far past this limit, and enemy stronger than fusion, Goku and Vegeta just might be able to do it.

Also in DBZ Ultimate Tenkaichi, there is a costume 2 for Gogeta that has silver fur and skin...this might be super saiyan 5.