“Why do we call quantum physics an ab initio model?
QM describes the behaviour of subatomic particles which make-up the matter. Solving a problem from this level is the ab-initio way.
Why do we call classical mechanics an ab initio model?
Classical mechanics describes the behaviour of our macroscopic way and not the way fundamental particles behave. So it can’t count as the ab-inito way.
Is classical electromagnetism an ab initio model (and if it is one, what are its first principles)?”
Classical electromagnetism does not employ concepts of quantum mechanics so it is not an ab-inito model.