Why are liquids usually much denser than gases?

Liquids are significantly denser than gases because of the fundamental differences in how their molecules are arranged and interact.

In the liquid state, molecules are packed closely together, remaining more or less in contact with one another. Short-range attractive forces cause these molecules to cohere, keeping them in a condensed phase. While they possess enough thermal energy to move past each other—preventing them from settling into the rigid, ordered arrays characteristic of solids—they are not moving fast enough to overcome these attractive forces and fly apart.12

In contrast, gases consist of molecules that are separated by large distances, allowing them to move almost independently of one another. Because these molecules are not in contact and are relatively free of each other's influence, gases have a much lower density and expand to fill any available volume. For instance, the volume of a given mass of steam is roughly 1,600 times larger than that of the same mass of liquid water.1324