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.
They already have a lot of experience as a distribution platform. I doubt the majority of Steam games even come close to a million units.
While it's not clear that there's enough market to extend that platform to commercialize their movie maker, it's not that far of a stretch from what they're already doing.
