Traditionally, hands free kits only provided sound through the opposite side to the driver - this was to cut back on feedback through the on-board microphone. As technology advanced, manufacturers were able to provide sound through all speakers by having a higher quality microphone that was unidrectional - would only pick up sound directly in front of it from a certain angle. Hence why sometimes if a passenger tries to speak, you can't always hear them. It gives a clearer sound. Quite a bit of testing is done on the setup for this when the car is in prototype mode, to ensure it works. Perhaps Mitsubishi didn't spend too much time on this part, and opted for the easy way of speakers to opposite of driver only.
I believe MMCS is a Kenwood system, but stand to be corrected. I'm also not too sure how old the underlying system actually is. I get a feeling the general hardware is older than you might think, despite the software and looks being updated. Explains why it can be sluggish and not intuiative. The car play system will be new obviously, but it has been designed to fit all Mitsubishis accross the range, from cheap to expensive - so again, that's probably the easy way around it for why it's still passenger speaker only.
2019 PHEV GX5HS
Ruby Black Pearl