I'd wildly guess there's a bit more at play. Unless they add a converter in the game / editor, which would be a major task on its own, you'd otherwise have to use Wwise to convert music to .wem and make soundbanks out of it, and know exactly the format and settings this particular engine requires. Then there's the licensing ordeal with Wwise, commercial or non-commercial (how many sounds your project can have), adding some routine in the game/editor to update its current soundbank with the new files, and the whole grey-zone area of risking people using copyrighted music which could spawn a whole other legality issue. Perhaps there's some legality mumbo jumbo separating developers from mods in that regard, leaving consequences and infringements solely on the modder, I don't know.
That's just speculation.
