And again, if every single monster is so rare that only PCs encounter them, and only once or twice in their entire lifetimes... then why do the Adventurers fear those monsters? There are no goblins, hobgoblins, bugbears, ogres, trolls, girallons, displacer beasts, bulletes, phase spiders, ettercaps, owlbears, Blights, manticores, wyverns, griffons, hippogriffs, hags, Ankhegs, Basilisks, Bullywugs, Carrion Crawlers, Cyclops, Ettins, Zombies, Skeletons, wights, Wraiths, specters, ghosts, shadows, darkmantles, mimics, demons, devils, dryads, man-eating plants, ghouls, giants, gnolls, gricks, grimlocks, trogolodytes, lizardfolk, harpies, kobolds, kuotoua, oozes, perytons, yetis, peircers, ropers, Sahuagin or stirges out in the world.
After all, there might have been once, but the only known time of them being encountered, they were killed by adventurers and the entire region has been safe since then. And monsters can't just magically appear out of thin air.