At the end you are leaving yourself to the mercy of Big Tech that you “sell” out to. This is a question of trust. The problem is, if they have a lot on you, they control all this data and can do whatever they want with it, and they always have their interests in priority, not yours, and these interests are absolutely not guaranteed to always match yours.
Moreover, Big Tech already has extensive history of numerous mishandling of user’s data, which certainly doesn’t help in developing trust for them.
Even the wording that you use — “sell”. You do, consciously or not, understand, that there is a trade going on. You understand that you are the product. Doesn’t it bother you any bit?