(I had initially set out to write about the explosion in complexity of monster stat blocks compared to old school games, but that line of thinking led to something a bit more far-reaching. You never know what you're going to find when you start dissecting monsters.)
Monster stats sure have changed a lot over the history of the world's most popular RPG. Early editions took a pretty bare-bones approach toward a monster's mathematical DNA - mostly just some basic combat stats. When D&D "upgraded" to 3rd edition, though, I remember being completely flummoxed by the stat blocks I saw in my new issue of Dungeon Adventures. Suddenly monsters had a full complement of ability scores and skills, among other strange notations! (I let my subscription lapse shortly thereafter.) I'm completely unfamiliar with 4e monsters, but considering the ubiquity of the "skill challenge" mechanic, I'd be surprised if they weren't fully statted-up like characters too. Word is that monsters in the playtest version of 5e have ability scores as well.
I don't know what the initial impetus behind this seismic shift was. Perhaps somebody started wondering why monsters and characters seemed to operate on different mechanics, and sought to "unify" things. Perhaps it's merely the by-products of a misunderstanding of the importance of ability scores and the drive toward greater "realism" of the skills system.
In reality, in old editions monsters and characters actually did, for the most part, run on the same rules. Despite the obsessing of players and DMs over ability scores, they were never actually a core mechanic of the game, only a peripheral one. They provide some points of reference for imagining and role playing the character as a unique individual, and a few minor statistical deviations from the baseline in certain actions that were governed primarily by the real core mechanics. The real cogs and pulleys of the game engine were the things that were shared in common by PCs and monsters alike: a number representing how hard they are to hit (Armor Class), matrices determining their ability to hit (a function of levels for PCs, Hit Dice for monsters), a number representing how much punishment they can take before dying (hit points, derived from class and level for PCs, Hit Dice for monsters), a number or range of damage they cause with successful attacks, chances for avoiding or mitigating the effects of special attacks (saving throws, again a function of level/HD), and a movement rate. Some of these things could be modified by ability scores, but none were dependent upon them. The game, in fact, would play perfectly well without ability scores - essentially, as if everyone had modifiers of 0.
Thus, there was no point in generating ability scores at all, save for those entities most important to the game, the player characters. Take a look at the "monster" versions of humans and demi-humans (e.g. Bandit, Noble, Trader, etc.) in the rule books - nothing but standard monster stats. Or thumb through the Keep on the Borderlands module, and see how many of the denizens of the Keep had been given ability scores. (In a few cases, a single exceptional score is noted. Other than that...nada.)
In many or perhaps most cases, ability scores don't even apply very neatly to monsters. How do you rate a horse's Strength score on a scale meant for humans?
Do you give it the strength several times that of a man, as a real life horse would
have? If so, what do you do about the massive bonuses to hit and damage
that would come with such a score? How about a pixie? Or a grey ooze - does
it even have a Strength score, and if so, how exactly does the creature employ it? Does
it have Wisdom or Charisma? Does it have Dexterity? How do you handle a giant's
Constitution? It's clearly orders of magnitude sturdier than any human
by virtue of sheer size, but does that give it greater endurance and resistance to disease too?
No, rather than rate every ability of every monster on the same scale as player characters, their abilities are subsumed in their standard stats. The Strength, Dexterity, and Constitution of grizzly bears and kobolds
are reflected in their damage ranges, Armor Class, and number of Hit
Dice. There's rarely any need to individualize them further, but when there is, it can be achieved simply by granting a bonus or penalty to any or all of these stats, rather than mucking about with the intermediary step of generating ability scores.
Early skill systems were similarly peripheral to the essential rules of the game. Originally they were bolted on to mainly to graft on background stuff that had nothing to do with interacting with monsters and NPCs, and often little to do with adventuring at all. Formally granting a character the ability to rig a sail or weave baskets doesn't even touch upon the core mechanics of the game or any of the situations that they govern. There's almost never any reason why you'd need to know whether an orc or dragon can do either, and if some situation arose in which it is important - say, encountering a ship crewed by orcish pirates - you just assume they have the ability to do what they're clearly doing.
Somehow, though, the game evolved to begin incorporating ability scores directly into core game mechanics, rather than simply modifying them with a bonus or penalty. In case there's any confusion, resolving a situation by rolling a die and adding an ability score modifier is an example of the latter. Even without the ability score, the roll can be taken straight. A check of "1d20, roll under ability" is a simple example of the former - it cannot work at all in the absence of the requisite ability score. (Granted, the d20 ability check does appear in old school rule books, but it's generally suggested as a sort non-specific or catch-all mechanic in the DM's tool box, to be used at the DM's discretion for adjudicating occasional actions covered by no formal rule. As a tool for ad hoc rulings and resolutions, it seems no more objectionable than any other. Only with the intent to formally codify it as an official mechanic for resolving specified actions does it really turn malignant.)
Maybe this was an outgrowth of the inflated importance so often mistakenly assigned to ability scores, or maybe it was a conscious effort to actually make them as important as they were perceived to be. Whatever, chicken and egg. Once you have ability scores exerting a direct rather than indirect effect, you either have a mechanical double standard - a thing is done one way for PCs and another for monsters, which of course is fraught with opportunities for imbalance and abuse - or you have to assign ability scores to the monsters so they can use the same mechanics. If a certain action in combat is resolved with a check against Strength, then monsters need Strength scores, and even more problematically, they need Strength scores that are directly comparable to human Strength scores. Simply rolling 3d6 won't do - see the aforementioned examples of the horse and the pixie.
Similarly with skills. As long as they pertain to things like singing, basket weaving, and historical knowledge, they don't intrude upon the jurisdiction of the foundational rules for combat, negotiations, and other interactions, so they generate no disequilibrium if monsters don't have them. When they do start branching into those areas (sometimes morphing into "feats;" same concept, different label) they become a point of asymmetry between PC and monster, and the pressure to balance things out mounts. If a PC can have a Dodge skill for additional chance to evade attacks, what justification can there be for monsters not to have access to a similar ability? When skills and feats function on an opposed mechanic of some sort, it's pretty much mandatory that monsters have skills, else what exactly is the PC's skill opposing? When skills and feats are based on ability scores, it's then required that monsters have ability scores as well.
Where these changes lead in the end is not a to few new bells and baubles tacked onto a simple system, but to a fundamental transformation of a simple system into a much more complex one.