So, IMC.
Phaedrus said:
How do you explain the prevalence of monsters in your campaign?
Monsters have various origins. Some came from another world, or where modified by another plane.
But most of the "natural" monsters (i.e., non-aberrations, non-outsiders, non-undead, non-constructs) have been created by Spirits. Spirits are about the equivalent of small gods IMC. Each spirit is tied to a region and a type of creature (so they're kinda totems). For example, the Horse Spirit of the Kerchezel steppes is responsible for centaurs, unicorns, hippogriffs, pegasi, and other equine magical beasts and monstrous humanoids.
Spirits had no qualms copying the good ideas from each others, that's why you have so many chimeric creatures -- the aforementionned hippogriffs were made by the horse spirit when he copied the idea of the griffon from the cat spirit, who himself copied the idea of feathered wings from the bird spirit.
And they had no qualms about repeating their previous attempts and creating something else. Pegasi are refined hippogriffs, sphinxes are highly-advanced griffons, and so on.
Phaedrus said:
Why does your world tolerate the presence of beasties that threaten social stability, etc.?
Not all monsters threaten social stability. Usually, the various humanoid races (whose extreme diversity is also explained by Spirits) tame some monsters to fend other off.
The really dangerous monsters -- mostly aberrations and outsiders -- tend to come from other worlds, their impact is thus lessened by the long astral distance they have to cross.
An elite cadre of high-level arcane spellcasters, the Prime Order (which, by the way, are jealously keeping the secret of epic magic and preventing other sorcerers, wizards, wu jens and co. from discovering it without passing through their approval; they are about 40 all said, most of them not epic, and non-affiliated arcanists are all of a lower level) protects the world from these outside threats.
It doesn't make them good guys, necessarily. One of their most useful member, Ygrel, is probably the worst power-hungry selfish bastard the mortal races have spawned. But they're all aware of the menaces that threatens their own existence as well as the rest of the world.
Prime Order's second role is preventing spells of level 8, 9, and 10 (epic) from affecting the setting too dramatically. Clerics are kept under check by their gods, druids by their ideal, but arcanists have no outside compunction to "play nice" with the mundanes. The Prime Order fill this slot.
Anyway, thanks to them and to other watchers (including some "monsters" such as sphinxes or dragons), machinations by illithids, kaorti, silians, malafides, neogis, fiends, aboleths, and so on are not overwhelming. The world's still gonna be destroyed unless the PCs complete the epic quest they don't even know they'll have to do yet

but at least, for most people, the looming threats upon existence itself is invisible. It's only as the heroes progress and unravel some mysteries that they'll discover the size of that Sword of Damocles hanging over them.
So, clueless mortal societies, at large, are not aware of any real danger.
Phaedrus said:
How does civilization--kings, cities, feudalism, trade, travel--exist alongside marauding monsters?
Feudalism exists, despite what magic allows for communication and travel, only because of marauding monsters. An ettin skirmish band, a sudden ankheg infestation, bulette mating season, orcish horde, goblin marauders, and the odd juvenile green dragon bully, all of these are valid reasons to keep an overall inefficient social system; because said social system, for all its political and economical shortcomings, make sure that you'll always have a nearby castle with a batch of skillfull knights that can protect you from CR 2--8 monsters.
You have knights patrolling the roads and rivers to make sure the monster population is next-to-nonexistant around trade lanes and tended fields.