PCs and NPCs fulfill different roles in both the game system and the narrative. Therefore, they should be treated differently by the system. Also, it's typically easier.
A "class" is in and of itself a gamist construct. They exist for simplicity in the rules, not because they necessarily make a lot of sense from a purely simulationist standpoint.
So, for example, if you gave a monster powers and traits that were similar to fighter powers and class features, isn't it essentially a fighter anyway? You can still do this in 4th edition. A 4th edition monster is exactly as complicated as you want it to be--no more, and no less. I like that the rules support making both highly complicated monsters that might as well be PCs, and a minion who might show up in the campaign for a grand total of 2 rounds of combat. I'd like it if DnD Next continues that trend.
For the record, I homebrew 4E monsters all the time, and I've made some viciously potent and complicated monsters. Occasionally I have given them powers that the PCs have access to as well. I don't feel limited at all in what I can do in 4E, and I can do it quickly and efficiently if I need to (thank you Adventure Tools).
Response to the first part: Not always true and it depends on the person who is DMing. This is all part of playstyle and DMing style. I can tell you right now that I like for me NPC's to be fully fledged out. If they are able to do something then I want that mechanic to be in their write up. If they know a certain amount of "forbidden lore" then I want their stats to reflect that fact. I don't enjoy adding in things for the hell of it or just having them know a certain piece of info "just because".
Response to second part: Give me an example of your "complicated" monsters.