Why can't liquids change volume but gasses can?

Because liquids cant be condensed the way that gasses can for example in a tank of argon you can put 20 cubic feet because it can be be condensed but you could not fit 20 cubic feet of water because it can not be packed together . 

Gases don't have a definite shape or volume and fill any container. and since the particles are far apart from each other their attraction is weak. since there is a lots of free space between particles, gases can easily be compressed. water particles are  held very close together by strong forces of attraction."Because the particles are close together, liquids can't easily be compressed and keep the same volume."

hope this helped