Once you hit city size there are enough high level NPCs that only an army with an equal number of high level NPCs, a thousand year old dragon, or a loose high level fiend can threaten it.
Most of the "maneating" monsters are restricted in living space. Many undead are powerless in or just don't like the sunlight, and thus stay in their ruins or underground. Aboleths live in underground waterways. Mind Flayers live underground. And on and on. Furthermore, there's something in dnd we don't have in real life: cattle-tier humanoids. That is, kobolds and goblins which are fully functioning humanoid races which grow twice as fast as humans (6 years for kobolds), and hey look they often live underground as well. So the maneating monsters mostly live underground and have easy access to less well developed cultures of food sources that grow faster than the more well defended surface dwelling humans. These two problems could be seen as sorting themselves out: the humans (and dwarves and elves) aren't overrun because goblin and kobold populations are kept down by predation.