[spinmaster]

This has one simple reason: Kodi does not have any folder restrictions for movie sets.

You have a clean setup where tmm could put the movie set artwork in the movie set root.

But e.g. my setup would be the following:

/movies/F/Fly, The (1986)/
...
/movies/F/Fly II, The (1989)/
...
/movies/F/Fantastic Four - Rise of the Silver Surfer (2007)/
...

where should tmm put the movie set artwork in this case? /movies/F/ would be definitely wrong.
Since Kodi has no restriction here, we do two things:

a) store the movie set artwork in every movie folder
b) store the movie set artwork in an artwork folder (if enabled in the settings)

Hope this helps

Thanks for making that clear... Keep up the great work!