Actually having worked in big companies,many of them just track everything, but don’t actually use the data, which is even worse.

That’s already part of the EU regulations people in the comments complain about

The most valuable data breach content isn’t your advertising tracking data, though.

It would be your payment information, which is orthogonal to most of the tracking data.

The black market demand for leaked advertising-related tracking data is basically nil, except maybe in cases where it’s related to something else exploitable or usable for blackmail like if someone frequents cryptocurrency exchanges or porn sites. Nobody cares to pay for black market data about you shopping for towels on Amazon or things like that.