Why can't liquid change volume but gas can

Because the molecules in liquids are more tightly packed than gases, they are restricted to their container. Gases, on the other hand can move a lot more freely due to the extra space.

Particles are far apart from each other because there are very weak forces of attraction between them. Because of this, gases don't have a definite shape or volume and fill any container. Because there is lots of free space between particles, gases can easily be compressed.