Here's why:

* Closely packed molecules: Unlike gases, which have molecules far apart, liquids have molecules packed closely together.

* Strong intermolecular forces: The molecules in liquids are held together by strong intermolecular forces (like hydrogen bonds or Van der Waals forces). These forces resist any attempt to push the molecules closer together.

While liquids are difficult to compress, they are not completely incompressible. They do have a small degree of compressibility, but it's much less than gases. For example, water can be compressed by about 5% at a pressure of 100 atmospheres.

However, for most practical purposes, liquids can be considered nearly incompressible.