Explain why sodium chloride solution is regarded as a mixture while sodium chloride itself is a pure substance

Because :

Mixtures are constituted by more than one kind of pure form of matter, known as a pure substance. Sodium chloride solution is regarded as a mixture since dissolved sodium chloride can be separated from water by the physical process of evaporation.

A pure substance cannot be separated into other kinds of matter by any physical process. Sodium chloride is a pure substance; it cannot be separated into its chemical constituents - sodium and chlorine - by a physical process.