I don't claim this to be any more than a personal anecdote, but I thought it was interesting the other day that while perusing the Races section of 13th Age I had absolutely no interest in reading about humans. Similarly, more often than not I tend to avoid humans while building characters for D&D as well. Now, this was interesting because just a few days earlier I was working on a character for The One Ring, and I found that I was drawn most strongly to the human cultures (Bardings, Beornings, and Woodmen). Granted, I'd also LOVE to play a Hobbit in that game, but I digress.
Perhaps the broader issue is a desire to have tangible ties to the greater world in the game. The human cultures in TOR exemplify this beautifully, not surprisingly, as they're all very much a part of Middle Earth. They're all human, yes, but they all have different mechanical bits to distinguish them from each other (which also happens to fit their flavor exceptionally well).
In D&D, 13th Age, and most fantasy RPGs for that matter humans are typically generic. Their schtick usually seems to be "they're diverse and adaptable," and while this opens you up to a lot of different possibilities it also gives you absolutely nothing to go on. You're starting from scratch, as it were, which is fine if you have a very specific concept in mind, but that starting point is sometimes quite useful. If I decide to play a Wood Elf I know that I'll likely be a sneaky, perceptive guy who lives harmoniously with nature and is either very long-lived or outright immortal. If I'm playing a Dwarf I'm an ale-swilling tough guy with a Scottish accent who is really proud of his beard. Etc. Even if you decide to play against type to create an interesting and memorable character the fact still remains that the stereotype is a starting point.
What it boils down to is as soon as someone says they're an Elf I have an immediate image in my mind of the culture that that character comes from. Same with a Dwarf, and same with a Beorning, Barding, or Woodman. But a generic Human? Not so much. For a genre that's so heavily influenced by archetypes, this is significant (whether that's a good or bad thing will depend on the player, though!).