I seem to recall that they clarified this at one point. None of the races, classes, or backgrounds, are core. The only core thing is that races, classes, and backgrounds exist; and they interact in a pre-defined way. You can't assume that a setting has humans, any more than you can assume it has gnomes or warlocks.This. If this thread was "PSA: Humans are core" people would be like "Yeah, and...?"
At least, that's what I seem to recall. It could have been one of their propaganda lines, from before the books were printed, just like the idea of supporting the 4E-playstyle.