They are making FFVIIR as a multi-game thing because it's impossible to make it as a single game (in a timely manner with a budget they could recoup with a single 60$ game without the game having to sell 20 million copies) if the remake is to not be a completely ripped to shreds version of the OG story/game with huge chunks taken out of the experience (there will still obviously be cuts in the remake we ARE getting, but it will also expand most everything that will be in the game). There was no other option if they wanted to make it in the way that the PS3 tech demo teased 11 years ago, except with even more powerful hardware, which makes it an even bigger project. Midgar alone, in its expanded form, is probably a bigger undertaking than a lot of whole games and that's only the intro to FFVII with a whole world filled with tons of more or less exotic & varied locations (+ every overworld field/area connecting those) and dozens of hours of story waiting past that.
FFXV being made into a single game and the decision to make FFVIIR into more than one release are because of completely different reasons that don't contradict each other at all. VsXIII/FFXV is something that had perhaps become a bit too bloated in its plans in the long years since its announcement, so instead of dragging players along the VsXIII/XV hype train even longer for another decade with sequels, it was probably a better choice to condense that into a single game. FFVII is a known entity. There are expectations as to what that story & game needs to offer. Any changes will be met with resistance, let alone completely truncating the game into, like, only 40% what the original offered.
I mean, imagine a situation where execs are deciding whether to make Lord of the Rings into a single 1,5-2 hour movie or a single 12 hour movie. Choice #1 is something financially feasible, but it would lead to a product that would truncate the OG experience into something almost unrecognizable & inferior considering how much they'd have to cut & change the story. Choice #2 isn't financially feasible at all (not even considering how horrible it would be to have to sit at a movie theater for 12 hours) since they could only sell movie tickets once yet the budget would be that of several movies and making a single huge project like that and being forced to releasing it all at once can have production problems that can be avoided by dividing things into a few more manageable parts. Considering all this, the best answer is actually Choice #3, a trilogy of movies. Same applies to FFVII -> FFVII Remake. If it was to be a single game, it would be cut to hell and back since a single release, PS4 quality, "full" FFVII with modern game design sensibilities is just a pipe dream when reality kicks in.