It's not world design at all. It's game play. Monsters are rare or the PC races of the world are dead. PCs see a lot of them, because fun. It would be boring if there were nothing much to encounter and fight, so they see an abnormal number of monsters.
You are playing in a world. The world needs to exist and make sense or you really can't play the game.
Saying that monsters have to be rare or everyone would die can make some sense... but then makes you wonder why they exist in enough numbers to be the types of threats PCs fight all the time.
I mean, no one enters a Dungeon in this game and expects it to be empty. No one travels through the woods or mountains of their game world and expect to encounter nothing. But if monsters were truly rare, then they would actually expect there to be nothing. You can't world build and just say "well, threats are rare unless the PCs are nearby, then they are common" because that is not how world's work.
Ankhegs don't have hives and come by the thousands. There are very few at a time. And ants don't destroy towns, so why would something based on them?
Ants don't destroy towns because they are tiny, inch long at most. Ankhegs are the size of horses.
And how many do you think you need to destroy a town? Their acid spit is death for commoners, and at a 30 ft line is likely to hit a few. Plus they burrow and come up from underneath, and each attack from their bite is also death. HAve a cluster of six make a town their hunting grounds, and that town is dead unless they can hire people to track down the monsters and kill them.
I've never seen an empire by any of those races. Drow and Yuan-Ti make their homes away from PC race civilization, and Hobgoblins would live in the wild, so away from most of civilization
All three are famous for empire building, and it is listed in their lore. And all three are commonly known as slavers. Can't get human slaves if you don't attack human population centers, not in the numbers these races utilize.
Drow leading raids on the surface is actually specifically called out in their lore entry.
Cool. There aren't 3 just wandering around, though. There's probably not even 1 wandering around.
Because somehow being specifically summoned to slaughter a village somehow makes them less of a threat than them wandering through the wilds?
I think you are overestimating how many monsters are in the world. Perhaps because there are lots of monster books. If monsters were as common as you are making out, there would be no civilization left.
That is one assumption you can make. But it is not an assumption supported by anything in the texts. Far more likely is that there are a lot more defenses, guards, and powerful people in the world to protect civilizations. Most towns would have a dedicated and trained guard force, defenses built to help protect them from the more common threats of their area.
No, it really isn't. It's not "full" of them at all. They have to be very rare near civilization in order for civilization to have survived.
Again, that is your assumption. It is not supported.