Question

Why liquids and solids are hard to compress?

Answer 1

Liquids and solids are hard to compress because, in liquids,
the molecules are tightly packed when compared to gases.
In solids, the molecules are very tightly packed.
Therefore, they are hard to compress. Whereas gases are more loosely packed.

They can be compressed easily.

Answer 2

⇒The molecules in the solid's and liquid's are tightly packed and they have less gap in between the molecules. When compared to that of the gases, in the gases the molecules are loosely packed and they have a lot of place in between each of them.  
⇒Hence it is hard to compress liquids and solids when compare to gases.