New Job: Viper

[Duskane]

probably the only thing that annoys me how in game the swords are on your back but in the cinematic its on your sides

[Abbie_]

probably the only thing that annoys me how in game the swords are on your back but in the cinematic its on your sides

different weapons might sheathe differently, like sage

[Mimilu]

different weapons might sheathe differently, like sage

SGE can get away with that because their weapons float. With swords you have to grip them so the unsheathing animation is probably going to be the same for all Viper weapons for ease of implication.
My guess is they put Viper's swords on their back because NIN's weapons are on their hips and they need to differentiate the two more. ( 　ﾟ,_ゝﾟ)

[CKNovel]

probably the only thing that annoys me how in game the swords are on your back but in the cinematic its on your sides

This is my biggest gripe as well, swords on the back isn't cool and even less practical.
Swords on the waist is much better and fits with PLD/NIN.

[Shurrikhan]

This is my biggest gripe as well, swords on the back isn't cool and even less practical.
Swords on the waist is much better and fits with PLD/NIN.

There are cases where a back-slung scabbard is the more practical just to keep out of your way. You see this more often in people who have to deal often with considerable slopes or just using far longer weapons in general, especially if wearing a backpack already (which then reduces the variation in its resting angle as you have to bend during climbs or descents).

But, you don't take a scabbard meant to release its sword through a constrained pull and place it somewhere that's impossible to do with any but the shortest weapons. That leaves you with two options.

The cheaper and far, far, far more common is to just also carry a shorter, less cumbersome weapon on one's hip to use for actual emergencies, and then sling the back-worn weapon off, unsheathe it, and toss the scabbard aside to pick up again after their fight. Even caught off guard, especially if you had anyone to take some pressure off you, you could usually thumb the strap on your back-slung scabbard to get it widened or even fully released while still using your sidesword.

The other is a scabbard actually designed to be used from the back. That, however, is a whole different beast, requiring that the blade can be released through the side of the scabbard, usually through a pressure release (pull out slightly, pull sword against the inside edge of the top of the scabbard to pull on a bit that releases the catch from the other side of the scabbard, etc.) or something similar.