Because (in some circumstances) it would break the game? An NPC who can both do brilliant weapon play and brilliant spellcasting is interesting. A PC who can do both is (frequently) overpowered. An NPC who gets free attacks against enemies starting adjacent to him/her is a viable foe against multiple PCs. A PC who has such an ability is (frequently) overpowered. Etc.
There's no reason why it would break the game.
An NPC who's brilliant at both swordplay and magic (as good as the party fighter in melee and the wizard in a spellbattle, say) is simply higher level than the PCs. I would say obviously. Since he's managed to hone his skills in two separate fields enough to match a resourceful, battle-hardened specialist (i.e the PC), he's clearly got more experience, more intensive training or both under his belt.
And then you don't need to contrive powers that exist only for gamist reasons to make him hold his own against the party in a fight, it comes quite naturally.
You don't
start with spesifications for a challenging encounter and design how the world works from there. You start with what's in the world, then design challenges using those elements. That's why I don't think 4e has a reasonably good system for designing encounters. Or rather, it does, but it gives up far too much to get it, so you end up with a game that for my purposes is total garbage.
But there is no obvious reason why they should be built using the same rules. As I posted upthread, even games that are very austerely simulationist in their PC building rules - Runequest, Classic Traveller, Burning Wheel - don't go this far. The only non-points-buy game that I'm aware of that does take this approach is 3E. (And in points-buy, of course, you can just give the NPC/monster the number of points you need to get it to do what you want it to do. So the currency becomes somewhat meaningless outside the context of PC building.)
The obvious reason why they should be built using the same rules is that from an in-universe perspective, there's not supposed to be any way to tell a PC from an NPC! As a corollary to this, if you use the same rules for everything, it's a lot easier to handle situations where characters end up doing something other than what you've designed them for.
My example got kind of long, so I spoilered it:
[sblock]For example, in my last 3e campaign, in an early adventure the party fighter ended up sparing a random kobold warrior's life and taking him on as a henchman. Because he worked exactly like a PC, it was no problem to have him tag along, define what he could do or learn besides stabbing people with his shortspear, calculate how much XP he got from the adventures he participated in and decide how he would advance.
In 4e? Most likely he would have been a Minion, and having him do anything at all besides soak up attacks would be an exercise in frustration. Even if he were a normal monster, he would stick out like a sore thumb in the party with his monster statblock. If I converted him to PC rules or something like it, the kobold warrior they fought would end up totally different from the kobold warrior who joined them. Instant loss of immersion.
In another game a PC got transformed into a dragon as part of an ill-worded Wish. How would you even do that in a game where a dragon, as an NPC monster, has a completely different kind of stat-block than a PC? Would you keep the PC but give him some kind of "dragony" powers? Then what when the party runs into a real dragon with entirely different powers?[/sblock]
The way you think about points buy - as metagame - is the way I think about class and level. Otherwise, I have to posit a world in which everything in human development is tied to everything else.
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I'm not going to defend the AD&D skill system. I agree that it creates bizarre results and I would not use it myself.
I agree that the motives of the designers of the PHB in assigning feat prerequistes were probably mostly concerned with game balance and not simulation (although there's some element of simulation too, otherwise why would Dodge require a Dex bonus or Combat Expertise an Int bonus? They're hardly the most powerful feats).
That doesn't mean that I can't interpret what they made for my own, simulationist purposes and conclude that, e.g, Whirlwind Attack is a highly complicated technique that only veteran combatants with great quickness and technical skill can master. Which doesn't even invalidate the metagame reasons for having those prerequisites in place! PCs who can attack multiple foes will be too strong below a certain level
and, if not
therefore, Whirlwind Attack is a highly complicated technique that only veteran combatants can master. The rules inform the fiction. In fact, the book seems to bear this out, since the other, more straightforward ways to get multiple attacks (Two-weapon fighting, monk abilities, rapid shot, playing a character with natural weapons) ARE available at lower level, as low as 1st in several cases, but carry more drawbacks and limitations than Whirlwind Attack.
As to "a fighter is a fighter" - is there no one in the world who is an adequate combatant (ie low level), has mastered plate armour (ie has heavy armour proficiency), but who knows little about fighting with polearms (ie does not have proficiency in all martial weapons)?
Is there no one in the world who is young, not especially bright, certainly not a duelist or assassin, but charming and skilled in many crafty matters? (This would be someone with 8 skill points per level without INT bonus, but not a rogue.)
There certainly should be, and in a class-based game there are two basic ways of ensuring you can create a sufficiently broad variety of characters: Either classes must be sufficiently spesific (and numerous) that you can find an appropriate class, or they must be sufficiently flexible. In my own games I chose the latter, since I like minimalism. I'm defending the structure of 3e here, not every spesific detail of the implementation.
Unless you go to full points buy, the class and level system will impose constraints which cannot be justified on an ingame, fictional basis - they must be metagame. Once you go full points buy, then the metagame character of the currency for PC building becomes transparent.
Any constraints
can be justified on an ingame, fictional basis. It's just a question of what constraints you're willing to deal with and what kind of justification you can accept. (I could probably play 4e in my preferred simulationist style, but the ingame reality would be so bizarre that I doubt it would be very enjoyable.) This is why, for example, it doesn't bother me that you can't develop your skills above a certain level without also increasing your BaB and HP. The level of skill that can be attained by low-level characters is high enough that I am comfortable saying anyone significantly above that is in some way of heroic stature.
(In fact in my home games I house rule a lot of the things about feat and class progressions that don't make sense to me, like the inability to have a skillful character who is not also a budding assassin, but that's not really germane to this discussion.)