Why are units used that are to express the densities of gases different from those used to express the densities of solids or liquids

If the units for gases were the same as the units for numbers would be too small to be convenient.

For example, the density of iron is 7.87 g/cm³.

Using the same units, the density of hydrogen is 0.000 0899 g/cm³.

It is much more convenient to express the density in numbers that are easier to visualize: 0.0899 g/L, and that is still a small number.