The difference that ownership would give is consent. Western liberal democracies are based on the idea that you can do what you want as long as all parties agree to it.

I willingly give my personal information over to a variety of firms knowing what they do with it, because I value the services I receive more. It's not your place to say whether that's okay or not, because it doesn't affect you.

Human trafficking + consent = immigration. Organ trade + consent = organ donation. Prostitution between consenting adults arguably should be legal anyway, and already is in many places in Europe.

And yes, there should be a practical way to revoke access to data. There are ways to accomplish this technologically (eg. capability-based security keeps the data within your possession and you export the particular query that an outside firm would use; federated learning lets them train machine-learning models on the data without the data ever leaving your possession). We just don't use them yet, for the most part.