Specifically, data is only worth as much as the buyer is able to exploit it to make money. Meta and Google have figured out how to immensely profit off of personal data - but that's because they own the largest ad networks on the planet.
Few other players can leverage personal data to this financial extent. More importantly, the players that can leverage personal data for immense profit also collect their own data, and so externally-sourced data is only of marginal utility.
And you see this in the discourse around this issue. We theorize for example that some grocery chain would "pay a lot" for personal data. But to what end? So they can offer just-in-time enticements to go to the store?
A lot of these supposed "applications" of personal data seem pretty flimsy, and even where they are genuinely useful, their monetary worth is marginal.
Data is worth something, but it isn't worth nearly as much as widely believed.