Watch "The Social Dilemma" carefully, and you will understand it.

The problem with Big Tech is not that they "selling your data". This wouldn't be that profitable to them. The problem is that they make more money by having tons of users and being able to build really accurate consumer behavior models, which then can be used to be used as a way to change mass behavior.

The most direct way is of course by having hyper targeted advertising, but this also affects people in many other ways in ways that are not to the benefit of the user: consider how the applications are designed to maximize engagement instead of being made to help users directly achieve their goals.

When you are the one paying for the service, companies succeed when they serve you better, and the money they can get from you directly is order of magnitudes more than what they'd get by trying to monetize your attention indirectly.