I'm going to approach this from two different angles.
First, "classes that always grant X abilities" is not an in-world constuct. You can have a lot of druids, and they may have similar abilities. You can call someone a paladin, but the concept that every paladin has every one of the PHB features and no more is not true in-world. Classes exist as a mechanic for the players of the archetype while in world they are a loser concept. That doesn't mean that the concept of "druid" or "sorcerer" doesn't exist - they do - they just describe the in-world fuzzy diagram of them, not the PHB specific mechanics. This is pretty official - look at the Druid in the MM, it has 4th level casting but no wildshape. The various other NPCs also do not exactly match the PC classes.
Second, I remember 3.x where monsters were built using the same sort of rules as PCs and it got very cumbersome for the DM at high levels. I want a chart that tells me these are the basics for a CR and how to play with them, like 4e, 13th Age, and 5e does. I mechanically do not want to have to build multitudes of PC-complex foes every week to play. So from a DM prep perspective, NPCs do not match PC creation rules.