How can clouds of ammonia float in Jupiter without sinking?

So, looking at CDE's density figures, hydrogen and helium are overwhelming lighter than anything else, particularly molecular gases. So, that made me wonder, why don't clouds of Jupiter or Saturn sink to the bottom right away and produce an atmosphere of pure hydrogen and helium? On earth water vapor can form clouds because there's plenty of other heavier molecules for the water vapor to float on, but gas giants are mostly hydrogen, and even supposed have lighter hydrogen sunk in their cores.