The problem is the clocks in the separate Pi’s (or any audio playback devices) are not synchronised.
Clock accuracy (from the crystal oscillator) means they may drift by about half a second every hour.
If you are standing in the doorway between two rooms playing the same file that audio offset will be very disconcerting.
Devices like sonos have a PLL adjustment on their clocks to force the devices to be synchronised to <1ms to allow multi-room audio to work well. They are expensive though.
Technically the Pi can do this from a hardware point of view, but as there is no standard API for achieving this across different platforms, upstream kodi will never support this.
You can get a similar effect from resampling the audio by very small factors (e.g. 0.01%) but that would probably upset the audiophiles. But in theory that scheme could be handled in a platform agnostic way and could be implemented by upstream kodi.