Question

why are gases compressible but not liquids

Answer 1

because there is much larger empty space between molecules of gases as compared to molecules of liquids.

Answer 2

The atoms, ions, or molecules that make up the solid or liquids  are very close together. There is no space between the individual particles, so they cannot pack together. The kinetic-molecular theory explains why gases are more compressible than either liquids or solids.