1) They need an excuse for having a user with both byakugan and rinnegan
2) if the rinnegan was in the eye, it'd look so weird, the byakugan is basically just a pupiless eye, so there wouldn't even be a notable change other than nerve popping on the sides of the eyes.
3)gives an excuse that nothing done in modern times is original. Shadow clone technique is derived from limbo, healing jutsu derived from Kaguya's healing properties (or naraka path), FTG derived from Amenotijikara, sealing jutsus are derived from original sealing tech, SPCT. Everything basically stems from rikoudo sannin and his immediate family, not even danzo is immune to this as implanting eyes on other parts of the body seems like this otsitsuki beat him to it.