Why is FF16 just reusing assets from FF14?

[Seraphor]

I don't think you understand what "reusing assets" means. All of these are new 'assets', because they were created for 16. They might be reusing some ideas or concepts though.

Nobuo Uematsu reused several melodies between FF's 6, 7, 8 and 9. (check out 7's The Nightmare Begins and compare it to the opening of 8's The Extreme)
Meanwhile 14 and 16's scores each feature Masayoshi Soken.

Liquid Flame debuted as an enemy in Final Fantasy 5, not 14, and is not the first enemy to be used across multiple games.

The first seven Final Fantasies all used the same font.

Churches and cathedrals exist.