Why can gases be compressed more easily than solids?
Well, for the simple reason that MOST of the volume occupied by a gas is empty space.........
And thus for a gas, compression of a given volume simply constrains the gas to a smaller volume in which the volume STILL dwarfs the actual volume of the gaseous molecules were they condensed.
This is possibly beyond your level of study (it is beyond mine now!), but physicists speak of the mean free path of gaseous molecules. For a reasonably high vacuum the
On the other hand, liquids and solids are dense, closely packed materials, with little opportunity for compression, and thus a constant (and characteristic) density are expressed.