Why is a gas easy to compress?
Because most of the volume occupied by a gas is EMPTY space.
One of the initial gas law postulates was that the volume of the gaseous molecules was miniscule compared to the volume that contained the gas. Pressure is a function of the gas molecules which engaged in elastic collisions with themselves, and with the sides of the container. Most of the volume was empty space.
Gases are thus (up to a point) compressible materials. Solids, and liquids, the condensed phases, are largely incompressible.