The point isn't that they're unpopulated, but that they feel dead anyway. The NPCs add nothing to the worlds, they're background noise with extremely limited interactivity who are just there to prove that Square can throw a couple of prefab models in one or two rooms per world when they feel like it. Meanwhile, the worlds themselves are cumbersome-- trading in the tightness and brevity of Disney visits in previous games, which were designed to advance a story, with aimless, plotless runarounds through empty environments repeating the same tasks and diluting every character interaction into stilted replays of the film formula or really redundantly scripted, Saturday morning cartoon level scenarios. ("Buzz, you are definitely not going to get corrupted by darkness. Stop worrying about that, it could definitely never, ever happen, I am telling you because I am your friend and as your friend I know these things.")
They're not especially fun or engaging to explore-- there's zero reason to ever diverge from the beaten path, the treasure chest and hidden mickey locations are not so much clever as they are buried under a mountain of assets-- and it's distractingly easy to get lost in them without ever becoming immersed. I don't care how pretty your snow plains and tree canopies are, if that's all there is in sight I can get the idea in half the time and maybe we could get one or two original worlds that actually serve a purpose.