Because both quantum physics and classical mechanics can be used to explain natural phenomena. Furthermore, they both provide this explanation from fundamental laws, they are the basis from which you start. For classical mechanics, there is a certain amount of things you can explain this way (e.g. apple falling from a tree). Things that are moving too fast, that are too small,… to be properly explained by classical mechanics, can be explained by quantum physics. Electromagnetism isn’t an ab initio model because Maxwell’s laws (basis for electromagnetism) come from for example QED.