Why is compound a pure substance ?

Pure substances are defined as substances that are made of only one type of atoms or molecules
A compound is a pure substance composed of two or more different atoms chemically bonded to one another. A compound can be destroyed by chemical means. It might be broken down into simplercompounds, into its elements or a combination of the two.
In simplest terms, a pure substance is any material that has a fixed chemical composition. That is, the formula unit for that material is always the same. Take water, for example. The chemical formula for water is H2O. Every water molecule has two hydrogen atoms convalently bonded to one oxygen atom. There are other possible combinations of hydrogen and oxygen (more on this in a moment), but those other combinations aren’t water. Whenever we talk about water, we mean molecules that have the fixed formula H2O.