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.