Rendering two fairly realistic models on the screen at once isn't a big challenge these days, if they're the only thing being rendered.
I doubt it's a performance issue: the 3D model of a rectilinear room has fewer vertices than used to represent a single earlobe. It most likely is just an unimplemented feature; they also could have "cheated" by chroma-keying a 2D-texture background or rotoscoped the avatars onto a another 3D room scene, but what's the point when its just talking heads?
Games often only have the highest detail models present in cutscenes, where the rest of the game can be limited, and the camera can be controlled to limit how many characters are visible.
Yes - they do, but you don't have to get fancy with the environment for 2 static avatars locked in place. What are the odds that rendering a room was dropped because it is allegedly GPU-intensive vs it being a feature that is not fully-baked for a public debut yet?
Totally agree that it’s a stylistic and presentational decision and not a computational limitation.