Thanks for this
@Faolyn. This is honestly, truly appreciated.
Reasons:
People play them. Maybe not a lot, but enough.
See, that's I think the crux of my disagreement. I don't think it's enough. And, I think it's a number that is dropping as years go on. Or, at least the percentage is dropping. But, I have pretty much zero proof of that, other than reading chicken entrails, so, I can't really argue with you.
The fill the everyman niche well, and people like them for that.
They're generic enough that it's very easy to stick them in any world with few or no modifications. Many other races--dragonborn, any planetouched, gith, minotaurs, kobolds, goblinoids--have enough differences or history that they don't fit in some settings well.
That's actually not really accurate though. Halflings in every other setting are massively rewritten from the PHB. Darksun, Dragonlance, Eberron. If halflings are included in the setting, then they are rarely PHB halflings, to the point of being pretty much unrecognizable as halflings. Kender aren't even called halflings. And dino riding plains barbarians is about as far removed from the PHB depiction of halflings as you can get. Whereas Eberron elves aren't all that terribly different from bog standard elves. Same as Eberron dwarves. While Dark Sun elves are quite different from standard elves - they do share more points than differences. And an Athasian Dwarf is basically a really dwarfy dwarf.
Again, you're comparing minor races like dragonborn, which have very short histories in the game, with one of the core 4 races. I mean, of the examples you gave, only the dragonborn and planetouched (tiefling) even appear in the PHB. We need to be careful to compare apples to apples.
They're traditional.
They don't take up that much space in the book.
More races can be added to the PH without having to remove them.
Traditionally though, no you can't. 5e is the first PHB to add races. 4e added and subtracted. 3e and earlier never added anything that wasn't in 1e PHB. Five editions of the game and we've only seen one PHB that added races to the PHB. So, the idea that we can just add more races, while true, certainly isn't borne out by experience. And, three pages in the PHB is a LOT. That is a significant amount of space. It's not like the old days when a race description took up a couple of paragraphs.