Here's why:

* Pure substance: A pure substance is a single element or compound. It has a fixed and definite composition. Examples: water (H₂O), gold (Au), table salt (NaCl).

* Homogeneous mixture: A homogeneous mixture is a mixture where the components are evenly distributed throughout. It appears uniform. Examples: saltwater, air, sugar dissolved in water.

The key difference is that a pure substance is a single chemical entity, while a homogeneous mixture consists of two or more substances.

To summarize:

* A pure substance is not a homogeneous mixture.

* A homogeneous mixture can be made up of pure substances.