If hydrogen and helium are lighter than air, why won't liquid hydrogen and liquid helium defy gravity?

Title says it all. If hydrogen and helium are lighter than air, why won't liquid hydrogen and liquid helium defy gravity?

Answer

Gaseous hydrogen and helium are lighter than air. Hydrogen, helium and air are close approximations to ideal gases, and for an ideal gas the volume of one mole of gas is about 22.4 litres. That means the density of an ideal gas is proportional to its molecular weight, so hydrogen ($M_w = 2$) and helium ($M_w = 4$) are lighter than air (average $M_w = 28.8$).

However you're asking about liquid hydrogen and helium, and liquids are much denser than gases because the molecules are much more tightly packed. For example the density of liquid hydrogen is around $68 \,\mathrm{kg/m}^3$ compared to air at about $1.3 \,\mathrm{kg/m^3}$. That's why liquid hydrogen doesn't float in air.

Incidentally, the density of liquid nitrogen (a close approximation to liquid air) is about $800 \,\mathrm{kg/m^3}$ so liquid hydrogen would float on liquid air.