Why is light a form of matter?
In simple terms, light is electromagnetic radiation, and is therefore composed of photons. Photons do not have mass nor do they occupy space, therefore they are not classed as matter.
It is true that the "wave/particle duality" idea on electromagnetic radiation (including light) is often taught, but that only demonstrates that in some ways light responds in a way similar to the way particles behave, and in some ways it responds similar to the way waves behave. It doesn't mean that light actually IS particular in the sense of small fragments of matter. It's not.