I've heard this question before, of why there is different terminology for PBR shaders other than (examples) "Specular" or "Diffuse" and so on. I will try explaining this way: It's the same reason why "inset" is different from "extrude" or the cycles "pathtracing" is not just called "raytracing." Quite simply, they aren't exactly equivalent. Same with metal vs dielectric, the shaders are calculated with different formulas and it's not strictly "traditional "diffuse vs specular/glossy.
