People do this because they get some value like looking at cat pictures in exchange and to some extend certainly also because they misjudge the value they receive as compared to the value they give away. If you only track users and don't provide some value in exchange, then nobody should opt in. And the value probably also has to be immediate to some extend, just promising to provide better services in a year after analyzing the data will probably not convince many to opt in.

Your example is not completely relevant. Facebook has been known to track people who are not actually Facebook users, meaning they never opted in.