The DM can not metagame, definitionally
The secret to writing (or playing) characters that are smarter than you are is that you can take your time coming up with what they do. Maybe in-game your character has a razor wit and would have a snappy comeback for any situation. Out of game you've got a list of pre-prepared retorts you can bust out as needed.
It may be less impartial than a book with no financial connection to its subject.
I don't know where this 1982 reference point is coming from. This is a history book about a game series, not a reprint of an 80s game. If you walk into a book store and pick a history book off a shelf, even a well researched, nice, hardcover, it's like $20-25.
It's a little sus that it's published by the same company that owns Traveller now. That said, I think $7 is not a lot of money for a book you're interested in. $60 for the physical is outrageous though.
Stick with Star Wars, they have nice, safe-for-work Jizz Music.
There's also Star Trek Voyager: Elite Force where they get transported to a ship graveyard.
Blast off and nuke the site from orbit, it's the only way to be sure.
So I guess that's actually several questions, and they each have different answers.
Why does combat feature heavily in D&D? It doesn't. Or at least, not necessarily. How much or little it features is dependent on your DM.
Ok, so why has it historically been featured heavily? Because of D&D's lineage. The game evolved mechanically from wargames, where combat was the whole thing, and thematically from works like Conan the Barbarian and Tolkein, where fighting monsters featured prominently.
Why so many types of monsters, then, if works like The Hobbit only had a half dozen or so? Because The Hobbit is a single story, whereas D&D is a framework for creating lots of stories. Maybe one short campaign or a campaign arc has as many monsters as a Tolkein story, but then you go on to the next arc, the next campaign, and you need something new. You can obviously recycle lots; orc bandits are different from orc soldiers are different from orc cultists. But with (tens of?) thousands of games going on continuously, year after year, there's always a demand for new content to slot in, and monster design is often a handy thing for DMs to outsource. Hence, there are a lot of kinds of monster because there is demand for them.
Humans are the real space orcs.
Are you asking why there are so many kinds of monsters, or why monsters appear so frequently in the campaigns you've played in?
Sometimes restrictions breed creativity, though.