Because there's no guarantee that those assets will be respected.
At any point the game maker can say "V1 assets aren't supported in V2". Or they can flood the market with "rare" items. Or they'll buy an item which was "stolen" and have it unilaterally revoked. Or... the list goes on.
There's also zero possibility of cross-game use. If ID releases the BFG9000 for free, is Skyrim going to let you shoot that at dragons?
But, here's the thing, don't listen to me. Listen to your (potential) customers. If they're all telling you that they don't want to buy what you're selling then take that as a sign.
Just to expand on your last point, I read their ad copy and once you strip out all of the meaningless filler text you have this key quote: “with the permission of the developer”. If they weren’t going to do it before, this can’t force them and if they were, they don’t need it.
As a game buyer, that sounds like doubling the number of companies who need to be paid and whose business decisions or failure will break things, add additional fees, etc. “Our incredible journey” is another way to say “you’re going to get server errors if you try to use your purchases”.
As a game developer, that sounds like I still have to do almost all of the work but now I’m locking in payments and giving up some control of my business to another party whose interests only partially correlate with mine or my partner developers’.
In both cases, there doesn’t seem to be enough value to support a sustainable business. Any two game developers are going to need to collaborate to share anything between games, and there’s a really upper bound on how much money they’ll pay before cutting out the middleman starts to look really compelling. Similarly, many gamers don’t like pay to win and won’t pay anything for cosmetic items so you’re chasing the percentage of people who both do and are willing to pay more upfront and more later despite not even knowing what future games would offer that option or how much they’ll charge.
At any point the game maker can say "V1 assets aren't supported in V2". Or they can flood the market with "rare" items. Or they'll buy an item which was "stolen" and have it unilaterally revoked. Or... the list goes on.
There's also zero possibility of cross-game use. If ID releases the BFG9000 for free, is Skyrim going to let you shoot that at dragons?
But, here's the thing, don't listen to me. Listen to your (potential) customers. If they're all telling you that they don't want to buy what you're selling then take that as a sign.