Ok so sales are what they are, the question is WHY do you think they are what they are now?
Remake sold well, something around 700k first week in Japan. So why is rebirth so much lower? Few theories:
-The ps5 installed base isnt high enough yet, but that doesnt explain why it sold less than 16.
-The episodic nature of the game. I think this could paly a big role in it. Maybe alot of people who got part 1 didnt realize it was midgar only and that made them not want to continue with it anymore? Perhaps Squares lack of transparency when it comes to how save data will transfer from part 2-3 is factor as well (albeit extremely minor)
-Taking 4 years for each chapter makes people lose interest: Honestly this is probably the main reason. FF7s main audience is probably of a certain age group from the PS1 era and that age group has either moved on in life from gaming, or has a family now and doesnt have time.
-The demand for an FF7 remake is an extremely vocal, online minority that doesnt exist in large enough numbers in reality? Not sure this would be the case because part 1 did do well.
-Story changes/Whispers/Not direct remake: I could see this also rubbing people the same way. I dont think changes are all that dramatic in the big picture but from what spoilers ive read of rebirth, it seems they really walked back their ambitions to make whispers change things which indicates to me they got alot of negative feed back about it. Maybe this is a big reason as well?
Im sure i missed a point or 2 but it I think it could be a combination of all of these.