[mlaggner]

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