Hussar
Legend
Eberron would like to have a word with you.Sure, the real world only manages to support one sentient species. Two is fantasy, ten is ridiculous, and fifty (snice most of the species in the Monster Manual are sentient) is D&D. But the only way it works is if you assume that most of those species are very very rare. There isn't room on the planet for each one to have towns and cities.
Hell, Tolkien would like to have a word with you. Elves, dwarves, orcs, men, hobbits, and goblins all have their own towns. That's six right there. And, by and large, that's considered the gold standard for fantasy settings.
There are TONS of fantasy settings out there that have dozens of species that are neither hard to find, nor rare.
The point is, the option should be there. Whenever settings are created for D&D, they always incorporate whatever is in the PHB at the time. Greyhawk was designed before D&D had really nailed down the races, so, halflings and gnomes are a bit left out in the dark, but, there are orc nations and various humanoid nations in Greyhawk. When Forgotten Realms became a thing, the sort of PHB standard races all had places. Same as Dragonlance and Eberron and pretty much every single setting (barring may Dark Sun which replaced the standard races with new stuff - but certainly not less stuff).
Remember, we're talking about a hypothetical new setting. I'm honestly not understanding why anyone would think that a new setting for D&D would not have all the PHB species baked in. That's just not how it works.