Spiderman 2018 was bundled with the PS4 too.

As was FFVII Remake.

True, but bundling only gets you so far. I was honestly surprised they didn't do a PS5 Rebirth bundle, but the console was widely available at the time so if folks really wanted to play Rebirth but didn't have a PS5, they could have bought one.

I played FF7 back in the day and I think it is simply the best RPG ever made.
I also played 7 Remake and while I did like the added side content of the Avalanche members everything besides that just felt dragged and was either outright boring or too over the top (Honeybee / death scenes that took like forever).

After that I got the urge to play the original PS1 game again and what should I say, it still holds up excellently to this day. What a fantastic game! I did most there is to do and it just took me about 40 hours. A simple, tight and focused experience that I really like! Oh and lol, that's even less than what it took me to beat the drawn out Remake.
So after playing the original again I have no desire whatsoever to play Rebirth, a game which looks to be even more drawn out and unfocused than Remake was.
(Also, there is always the looming threat of too much "Nomura"-ization...)

Imo you can't name the first entry of a series "Remake" and the second "Rebirth". Most people (who haven't played the original game) should think that the first entry is a complete remake of the original game and Rebirth is a spin-off. Even "Remake - Part 2" would have been better.

The name is pretty messy.

I'm not saying something weird like SE did before would be great. Outright calling it Final Fantasy VII-1 (because it's a remake). VII-2, VII,3 would be a big mess.

Final Fantasy VII Remake Part 1 Final Fantasy VII Remake Part 2, and Final Fantasy VII Remake Part 3 would be fine.

Yes, the naming convention is nonsense and part of the "Nomura-ization" one of the earlier posters spoke off.

Sales are definitely lower than projections I'm sure, it not only moved half the copies of FF7 Remake in the same timeframe but also sold at least 1m less copies in the same timeframe than FF16 sold last year (FF16 sales were themselves down compared to FF15), and FF16 released to a smaller PS5 install base than FF7 Rebirth.

Several factors at play here I'd say:

1. $70 is alot to pay for any game in this economy. Stats show gamers increasingly waiting for discounts and playing older games over newer games more than ever before.

2. It's the middle game in a trilogy of remakes, many people are probably waiting on all 3 parts to release before they play FF7 Remake trilogy

3. Story changes in FF7 Remake proved contentious among the FF7 fanbase and may have chased away some FF7 fans from buying Rebirth

4. FF7 Remake released to a much larger late life PS4 install base than Rebirth releasing to a mid life PS5 install base.

5. It released fairly close to other big PS console exclusives like Helldivers 2 and Granblue Famtasy Relink, not to mention Q1 was jam packed this year in general, with other notable releases like Tekken 8, Like A Dragon 8, Persona 3 Reload, and Dragon's Dogma 2. Gaming budgets are more limited than ever, releasing in a busy quarter hurts.

PS timed exclusivity will have hurt sales for sure, but doesn't explain why it moved less than FF7 Remake and FF16, which were also both PS timed exclusives. PS timed exclusivity moreso explains why FF16 sold less than FF15, than it explains why FF7 Rebirth sold half of FF7 Remake.

But there is no doubt that PS timed exclusivity has hurt Square Enix on multiple games now, there is a reason why the new President who took over middle of last year seems to be moving them to day one multiplat releases going forward, starting with the new Mana game releasing on PS5, Xbox Series, and PC day one (with a Switch 2 port likely after Switch 2 releases next year).

Something tells me they saw Sega and Capcom having massive success having their games multiplat and asked themselves, "Why aren't we doing the same thing?" Some people go crazy just mentioning games should go multiplat (especially if you mention Xbox), but the reality is games available on multiple platforms benefit the company and the consumer. Everyone wins.

With that being said I'm certain FF7R Part 3 will be PS exclusive if they already made the deal for the whole trilogy. FF17 however, I would be shocked if it wasn't multiplat.

Yeah with games costing more and more to make, third party exclusives make increasingly less sense, which is why they're becoming rarer and why even Sony/MS first party games come to PC now.
Rebirth doesn't seem to have done much to boost PS5 hardware sales either, so while the deal for the whole trilogy was likely done years and years ago, and may have seemed like it made sense at the time, it's possible it may not have been worth it from Sony or Square's perspective in retrospect.