I'm pretty sure you can use the tradeable assets in your films without owning them. You only need to have them in your inventory if you want to use them in the game proper.

Yep, they're a mass-market company, selling millions of units at sub-$50 prices. The bestselling resources will probably sell a few thousand copies, and most will probably only sell hundreds. It may be more cost-effective for them to focus entirely on the mass-market side, where they've proven they're extremely competent (among the best in their industry), and let others figure out the smaller volume stuff. But, maybe a marketplace would be cool/profitable. They have begun to move in that direction for third party game add-ons, though I think they're all free, so far, there's nothing stopping them from adding them to the store.