Why doesn't Goku use the Kaio-Ken attack while he's a super Saiyan?

Answer 1

Cuase he is stupid and doesnt know any better

(Second answer)

Actually, think. Super Saiyan requires a saiyan to push themselves really really hard.

Which means it takes up a large amount of energy, while Goku still has to fight

and being at such a high power level takes up energy as well.

Now imagine if Goku is at Super Saiyan 4. Goku is at his peak.

Then imagine Goku using Kaio-Ken in that form. Yes,his power level would be impossibly high(well practically everybody in the dbz universe is like God), much higher than it is at Super Saiyan 4, but Goku would get tired much faster.

Then he loses energy.