Question

how is homogeneous not considered a pure substance

Answer 1

A pure substance is only one thing (like pure oxygen gas only has oxygen molecules in it).

A homogenous mixture is only an evenly mixed mixture, like when you mix salt into water, you don't see a load of salt at the bottom, it dissolves evenly in the water. If i were to take any sample of the salt water, i'd find the same amount of salt and water, but it's not pure because there are two things in there.