In my mind, the idea of dozens of sentient species all sprouting up around each other on the same planet doesn't necessarily make a whole lot of sense. Look oat our own. Several precursors, several pretenders to our throne of dominance that we either ate or mated with, and we're the latter day result. Numerous racial divisions exist, but we are all homo sapiens. Bipedal, omnivorous mammals that have proven to be highly adaptable to whatever surroundings we happen to wander into. That's why humans are all over the damned map, even in otherwise uninhabitable areas. We're tough suckers with a stubborn streak a mile long. No other being on Earth comes close. Arts, crafts, language, thought, and society separate us from the beasts of the wild. We are alone on this blue marble.
Since we pretty much killed or humped our opposition into oblivion, it seems unlikely that other higher life forms would rise to the same level at the same time. This also points to the idea that everyone "close" to doing so was of the same genus ( Homo erectus, Neanderthals, etc). Are "demi" humans just other homo species? That could make sense. Add magic to the mutation mix and I could see hominids splitting off into wild directions to match their environment. And that would explain why most look vaguely similar and why crossbreeding is possible. Using this idea, perhaps these could all come from the same planet and from the same common ancestor.
But what of other sapients? Mind Flayers and aboleths and beholders and so on? Well, we can always make those bizarre aliens from deep space, no problem. Prehistorical monstrosities from beyond time and space make me happy, anyway.
Perhaps animal-human hybrids (like raven men (kenku), lizardfolk, frogfolk, etc) are the result of mad science. That would work. Again, it would explain why so many beast-men types are at least somewhat cosmetically similar to men. How many monsters are just 1/2 man and 1/2 other thing, anyway? Lots and lots and lots.
Still, in line with the likes of Conan, I like the idea that certain races are bizarre precursors, strange leftovers from a previous time. Ancient people who's civilizations are long fallen, who dwell in weird lands that eschew humanity. Yuan-ti and kuo-toa for a D&D example.
But, if the races are not so closely related, it would make sense if each races started on its own native world, where it evolved in just such a way to meet its own needs. Elfkin could hale from a light, airy world, low on gravity, clean, splendid, and bursting with magic. Dwarves could originate from a hostile high gravity world whose toxic surface forced them underground. The home world of the goliaths could be a vast, rocky land of thin air and unbelievably vast mountain peaks. Halflings could have originated on a small moon, fertile and forested, with enough bad things to encourage roaming and higher gravity to encourage short growth...or they're short to discourage giant beasties from eating them.
Halflings are the ewoks of D&D. Think about that.
I like both origin ideas. I like when things make some manner of logical sense, instead of "poof, a god did it." (Sure, I'm planning on creating such a setting with some people, but that's beside the point...) Sadly, both origins don't particularly work at the same time. Both explanations would, however, point a related campaign in a different direction, making for some interesting background material.
The first option (interrelatedness of humanoid sapients) leads me to think of an ancient, cosmopolitan world of super science. Ethics issues ("we've created all these beast folk and they're kinda pissed that we treat 'em badly...who knew?" "The warforged want rights again? Can't those jerks just be happy we give them jobs?"), religious issues ("If you made us, what is "god?") could arise. Truly ancient racial rivalries could arise, not unlike our own world.
Maybe civilization has risen only to fall again in a cycle so that the origins of each is shrouded in the annals a a lost Golden Age?
The second idea (otherworldly origins for all) points me towards a Star Wars like Spelljammer setting . . . or, perhaps the remnants of one. What are all these species doing on this one lonely planet? What happened to their home worlds? Did a doom of epic proportions befall the solar system? Did sudden competition for resources arise?
Both settings could have a Great Age. Both could just as easily lie in ruins. I do have a special fondness for ruining a perfectly good world . . . .