Because it's not financially viable to spend 6+ years developing a AAA multi-disc game with hundreds of staff and selling it for $60 or less.
Who knows what the console market will look like in 6+ years time.
You have to be careful with this reasoning. You can use it to justify basically any kind of monetization scheme.
Let's be clear, they're already accepting kickbacks from Sony for console exclusivity, they're cutting costs by using UE4, and they're even outsourcing labor costs by using CC2.
This is 100% about money. It might also reasonably allow them to spend more time on each part, but FF7 was never really about exploring some side street of Midgar. These are things they themselves decided to do for whatever reason, mostly because action games tend to play better when they have many "arenas" for the gameplay to take place.
Also, very few games rely on the $60 price point anymore, everything's Digital Deluxe +$10-20 for things that cost them literally nothing, post-launch DLC, Collector's Editions which have huge profit margins, strategy guides, etc, etc. So many sources of revenue.