Other people have suggested that the multi-part series model is the only way Final Fantasy VII Remake could ever be completed and I agree with them.
Final Fantasy VII has three/four disks. It had a development budget of $67 million in 2015 when adjusting for inflation, extremely large for a video game in 1997.
Adapting that behemoth to modern sensibilities, especially when you're a relatively small in-house developer like Square Enix (compared to massive titans like Activision Blizzard) who has to heavily rely on outsourcing to get your video game completed, must be incredibly cumbersome for the teams they have in-house. Logistically it's probably a nightmare, so managing development in smaller chunks might be the only way that the team can avoid from getting completely overwhelmed, especially with other in-house titles like "Kingdom Hearts 3" concurrently in development.
If Final Fantasy VII were a three-part series, that's three retail releases that successfully capitalize on the iconic "Final Fantasy VII" brand. It would maintain the IP's longevity in the market, and it would maximize revenue potential.
It also mitigates risk. Multi-part releases allow Square Enix to adjust future titles in response to feedback from the first one, while a major release upfront would not only take much longer development-wise, but if it failed then its sales potential would be severely truncated.