The point is that it is much easier to say e.g. playable goblins have a bonus to thievery, or playable pixies are childlike and happily dancing, than to assign those qualities to human subraces.
Are you sure? Think of this: add a humanoid race into your setting, that is great at being merchants, bankers and enterpreneurs (represented with skill proficiencies or other special bonuses and abilities).
Option 1: make it a human subrace, without ability score modifications, and without cosmetic features (no specific skin color etc.).
Option 2: make standard Goblins be such race, or a subrace of Goblins, or another demihuman race.
Which of the two options smells of anti-semitism?
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My wish about human subraces in the DMG is mostly to just have a small
system in the DMG, which really could better be a more general system for creating races and subraces, and that would probably avoid the whole possible "D&D has races so it's racist" accusation.
That doesn't imply that there must be example subraces provided. The DMG could provide the system, and leave each DM to use it as they see fit.
Even if examples are provided, the most important thing is that for the human subraces (because apparently nobody thinks that having elven or dwarven subraces is "racist" in the least) all references to physical/mental differences are avoided, as well as sensitive issues (as you mention, thievery is one of them, perhaps alignment could be another).
Still, there's a lot of room left: I don't see how having seafaring humans, mountaineer humans, nomadic humans, forest humans, desert-living humans etc. must imply to define skin color or facial features and ability score bonuses/penalties.
Design tip: presentation goes a long way, so if instead of presenting these as "human subraces" they would call them "human nations" or something like that, the issue would be already diminished.