I’m guessing because the vast majority of the market with a 4K Apple TV is not watching in stereo, but in surround with something like Dolby Atmos.
Edit: For those unfamiliar, spatial audio is a post processing effect that attempts to simulate surround, whereas most home theaters are going to be watching material that has discreet location and channel data, meaning a much more realistic surround stage.
Edit: For those unfamiliar, spatial audio is a post processing effect that attempts to simulate surround, whereas most home theaters are going to be watching material that has discreet location and channel data, meaning a much more realistic surround stage.