This is also why D&D, in the hands of a weak or mediocre DM and players, has such a dysfunctional and bland combat system - it is not in itself enough to reward intelligent play. (If anything, it does the opposite - if you have better AC and more hp and do more damage than the opponent then you will win, and the process is effectively mechanical. Just keep rolling the dice until you win.)
This is because, at its inception, D&D was not a game about combat. This is old news to folks who’ve been following the OSR for a while; if you look at the old games, you’ll see that the EXP you got for slaying monsters was a mere fraction of the EXP you’d get from their treasure-types. Toss in the reality of unguarded treasures and you have a game that rewards exploration, good mapping, clever play, and only engaging in necessary fights.
most of the rules were about combat and if you were coming to the game from adventure comics, it might seem like the coolest, most exciting things you could do with your characters revolved around combat. Computer RPGs only heightened this sense, since combat and wandering about were the only real activities in those early games. Later computer RPGs barely fiddled with that formula, focusing instead on graphics and interface, largely to make the combat aspects more compelling. The idea that your game should be about something else hardly ever came up.
Which meant that D&D was zigging while people interested in fantasy adventure were zagging. Early attempts to correct this resulted in 2e with its byzantine class-specific rules for earning EXP.
Things only got worse when WotC took over. By 1999 and the release of 3e, D&D was clearly about combat.
Yet the chassis of D&D was still heavily influenced by that earlier game where combat was not the central activity but a way of adjudicating failure. WotC-era D&D has in great part been an attempt to square the circle of a game that’s supposed to be about combat, but has a fairly “robotic” and extremely abstracted combat system.
And this is where you get the disconnects between Old School and New: the joy of rust monsters, the necessity of wandering monsters, and linear adventures (ultimate efficiency from every encounter being experienced wedded to superior story through creating rising tension from encounter to encounter) vs. non-linear adventures (where player choice, freedom-of-action, and exploration take precedence over efficiency and rigorous story structure).
These issues have profound effects in design and play. For instance, New School DMs work hard to make sure every encounter is balanced and interesting, because that’s where most of the play happens. Random encounters are eschewed as being time-wasters; they certainly are not feared by the players as monsters are EXP-on-the-hoof, and you must kill every monster you encounter to make sure you have all the potential experience (and levels, powers, and magical items) as possible before the big boss battle.
Old School DMs don’t worry about balancing combats. Since most combats are optional, players have the luxury of entirely bypassing certain fights if they don’t feel up to them. Also, Old School players have all sorts of abilities and powers (as well as the ubiquitous randomly-rolled rumors) by which they can learn about their foes before they fight, allowing them to pick their battles and prep ahead of time. Wandering monsters are justly feared not because they’re incredibly dangerous (they tend to be rather weak, in fact) but because they drain valuable resources for very little gain. Since treasure, not combat, rewards the most EXP, players are more eager to bargain with, evade, or simply trick the monsters they encounter. Where an inaccurate player map is devastating to a New School game, an accurate map is invaluable in an Old School game, and so dungeons are designed with an eye towards thwarting accurate mapping. Since combat in Old School D&D is about draining resources, rust monsters are an obvious choice for an Old School dungeon. For New School D&D, rust monsters are annoying in that they cause the PCs to retreat from the dungeon, which is rightly seen as a waste of valuable playing time.
So there you have it. Armed with this knowledge, you can now maximize the fun in either Old School (aka TSR-era) or New School (WotC-era except 5e kinda) D&D. Go forth and have fun!