Because while it's inspired by physics, it's usually a limited model that's not 100% physically accurate. "Physically based rendering" doesn't have to include p

Because while it's inspired by physics, it's usually a limited model that's not 100% physically accurate. "Physically based rendering" doesn't have to include photon mapping, for instance, which means caustics won't be visible (without some other way of cheating them into place, anyway).