The choice of algorithm is less baffling than the choice of mode of operation. When designing a new file system, why in world would you use unauthenticated XEX

The choice of algorithm is less baffling than the choice of mode of operation. When designing a new file system, why in world would you use unauthenticated XEX (or XTS) mode instead of an authenticated mode (SIV, HMAC+CTR, ChaCha20-Poly1305, Speck128+CMAC-Speck128 or whatever). It's not like you need a one-to-one block mapping between encrypted and unencrypted data — you design datastructures yourself, it's a new filesystem! Can't they afford additional 16 bytes?