Maybe it's just the difference between product families and product lines?

I always relate things to cars, so bear with me.

You have Ford's product line that has a my Crown Victoria. It also has the F150, the Mustang, etc. They all share some things in common because Ford reuses components and designs across all platforms in that line-up where it makes sense financially. So like the FF7/FF8 example, while there are similarities, they're not trying to be the same product. They're different products reusing assets where they can.

Then, also from Ford, you had the Mercury and Lincoln family or brand with their own product line.

Mercury had the Grand Marquis, and Lincoln had the Town Car, which are both essentially the same car as my Ford Crown Victoria. The three cars are far more similar to each other than any other vehicle in their own product line. The cars were marketed to slightly different customers, but the product was very, very much the same outside of cosmetic/customer interface differences.

Maybe in that way it's relates to how playing CT feels like playing FFs made around the same time with an ATB system and other similarities.

Meanwhile, newer FF's that are in the same FF line don't feel as related. Basically the studio is just tweaking what works/what R&D has already created, and offering it as a different product/family of products.