Question

why is a solid not compressible ?

Answer 1

Solids are not compressible because the molecules are tightly packed and the inter molecular space is less or is negligible . 

Answer 2

the solid is not compressible because the inter molecular spaces between the molecules of solid is very less or negligible, such that the molecules are tightly packed. since the molecules are close to each other, the solid is not compressible one.