Sodium and potassium are highly reactive elements because they belong to the first group in the periodic table meaning they have only one valence electron in their outermost shell and they need atleast 2 electrons in their valence shell(outermost shell) to become stable. So they react with most of their surroundings to take one electron and become stable. While kerosene is not reactive since it’s not a natural occurring compound, potassium and sodium are stored in it.