I'll admit I'm an old school D&D player/DM. I've never discounted a player idea in osr or 5e, but I still wonder. Turtle people (tortles) flying people (aarokara), dragon people (dragonborn)... and so on.
Why do people chose these races?
I'm a fellow traditionalist DM. (I avoid the terms "old-school" and "OSR" because reasons, just a personal choice.) And in my experience, at least in the circles I play with, tabletop gamers will do whatever they can to make their characters unique and special. Wanting to buck trends, flout convention, and be a not-normal special snowflake is almost an expected aspect of character creation these days.
Which makes trying to run a humanocentric setting
incredibly difficult if any playable demihumans are permitted. You can do whatever you want to the demihumans mechanically—make them clearly mechanically inferior to the humans, slap them with onerous perquisites and restrictions, limit what classes they can take (or just use the Basic D&D race-classes) and limit their maximum experience levels—and you'll
still get a table full of elves, dwarves, hobbits, and orcs with nary a human PC in sight.
So, I've found, if you want a human-centric game where the players don't have "I'm an elf! And my personality is—that I'm an elf!" to lean on as a character crutch, you have to make that explicit and just permit human PCs only. You'll get maybe five minutes of grumbling, and then the players will create interesting human characters with actual personalities to differentiate themselves from every
other human fighter, magic-user, thief, or cleric, and ten minutes into the actual game, they'll have forgotten that they had initially been meaning to play "loud drunken boorish violent Scottish-accented dwarf #276."
But.
The old truism always applies: "IT DEPENDS ON THE SETTING." Some settings are human fantasy. Some settings are Tolkienesque fantasy, with elves and dwarves in addition to the humans. Some settings are
Shining Force style kitchen-sink science fantasy, with centaurs and birdmen and robots running around, and humans aren't any more common or dominant than any other sentient species.
The rules—including any table-rules the DM makes regarding which races are playable in a given campaign—are there to serve the setting first, and it is
traditionally the business of the DM to do the worldbuilding and delineate the setting milieu.