The densities of gases are generally reported in units of g/L, whereas those for liquids are reported as g/mL.?

For common liquid substances, the densities fall in the range from 0.5 g/mL to 13.6 g/mL. It is easy to read when g/mL is used as the unit of density for liquid substances.

However, the densities of gaseous substances are two low. For example, the density of nitrogen gas at room temperature and atmospheric pressure is 0.0012506 g/mL. It is difficult to read such a small number. Therefore, the densities of gaseous substances are usually in g/L to make the number easy to read. For example, the density of nitrogen gas at room temperature and atmospheric pressure is 1.2506 g/L.

the density of carbon dioxide is:
D = 2.14 g / L
or
0.00214 g / ml

The density of water is :
D = 1.0 g / ml
or
1000 g / L

the traditional way that you refer to keeps the numeric representations smaller or more compact.

The molecules are spread far from each other in a gas. There is more space between the particles. In a liquid the particles are closer to each other.