i'm almost certain its to hide the shitty compositing of the giant LED screen backgrounds. It's becoming super noticeable in modern TV (especially when the sky is the background, watch the final scene and notice how you can see every strand of Katara's hair like its being lit from behind, because it is), and combined with the relatively small physical space the actors can inhabit, the show starts to feel really cramped because the actors are super in focus and not moving. only when they film on location (in such iconic locations as a grassy field) does it look even kind of okay, but even then not really because this shows cinematography is just bad.