A couple of reasons:Why aren’t humans also offered in a variety of ethnic/cultural/geographical variations? Boy would I ever wish they were, but I’m in a minority it seems.
- Humans are the default, the ones against which the other races are measured.
- Humans tend to get very flexible features. Variant humans get +1 to any two stats as well as their choice of both a skill and a feat. This represents human overall flexibility.
- Any attempt to say "humans from X are Y" comes close to some really ugly real-world implications.
- Offering multiple feats per region, usually 3-5. So it's not "Everyone from Sembia has the Silver Palm feat making them good at dealmaking", it's "some people from Sembia have Silver Palm".
- Feats show up in multiple regions. Silver Palm, for example, can be taken by humans from Amn, the Dragon Coast, the Golden Water, Sembia, Tharsult, Thesk, the Vilhon Reach, and Waterdeep (as well as assorted dwarves and halflings). So you can't be sure someone's from Sembia because they have Silver Palm.
- Many (but not all) of the feats were kind of on the weak side and had to be taken at level 1, so they were often ignored.