Like it was said many people get attached to these games and want remakes. There is also the elephant in the room that 3 doesn't play as well as 5R does gameplay wise.
Forgetting the visuals for a second, the persona series has evolved far past the roots of 3 and 4 and this way Atlus can get more people into persona while still not alienating fans because, "P3P is on PC already" which generally has all the content people want (honestly look up any discussion on 'The Answer' and you'll see why it doesn't need to be included). So fans will have an original-adjacent version and newcomers will have the new version closer to 5R. Which I'm guessing they hope both parties double dip