At the same time, humans, orcs, elves, manticores and cockatrices all inhabit the same world. They're all part of the same reality. Maybe there are "monsters" and they may not be strictly part of the ecology, but they're just sort of there in the same way that mountain is there or a tree is there or a jackdaw is there.
I'm getting less and less interested in this as time goes by. In fact I'm getting more and more interested in adventure as taking place in the spaces where the human world touches others. This could be the dungeon as mythic underworld. It could be the wilderness as mythic otherworld. It could be other planes of existence. It could be the realms of the gods. It could be the abyss. But adventure must happen when there is cross-over between different realities, in one form or another. That could be in a physical space, like when PCs venture into a megadungeon which is some version of hell. Or it could be temporal, like certain circumstances in which realms collide or overlap or rub together.
As a corollary of this I'm getting less interested in "monsters" in general, and more interested in antagonists as being spirits, demons, faeries. Not monsters, but things from other realities which are confronted because they have slipped into our own, or vice versa. Not parts of the furniture in the world of the PCs, however rare or powerful, but things that are qualitatively distinct.
This is the approach I'm aiming for with my Ainu Moshir game. When you're in the wilderness, you're not really in the world of farms and towns and villages. You're in a place that is other. You're in a space where you are an outsider. And the stuff that is out there is not tangible or understandable in any way that you have experienced before. It is a place which you can see or touch, but which you can never hope to be a part of. It is a place which is independent and which maintains its independence just as your world does. And just as you can venture into it if you dare, things from the wilderness can venture into your world - the world of farms and towns and villages. And just as you will never be a part of the wilderness, those things will never be part of your world. But they can see or touch it, in all manner of unexpected and frightening ways.