Ownership comes with both rights and responsibilities.
Platforms want as many of the rights as possible, without the responsibilities... which is why they have a contract (TOS) where they explicitly renounce to ownership, leaving it for the user, and only license the rights.
If platforms took full ownership, like in a "work for hire" agreement, they would be responsible for any illegal content a user could upload, since it wouldn't be the user's content anymore. Obviously they don't want that.
A side effect of wanting as much content as possible without owning it, is that... well, they don't own it. 😎
Fediverse where there's no owner/seller/buyer of your data or anything else you contributed.
Incorrect. You get ownership of anything that's yours, then upload stuff under whatever TOS your instance has... what's that? it has no TOS? Then they're in for a rough awakening some day. 🤷
Whether there are sellers/buyers... is something we'll learn in time. For now, user generated content on the Fediverse gets shared with little regard or protection of anyone's rights, so anyone can make a compilation, bundle it up, slap a price tag on it, and try to sell it.