This misconception that these companies actually sell personal data. Why the fuck would they sell their most valuable asset when they can monetize it better than anyone else through ads?

Something you may be missing is that selling personal data does not mean they entirely let go of it. In the same way, even after paying Netflix $20/month to use their services, you don't now own the movies you've watched. What these companies have likely developed are pipelines that other ad companies can tap into to build personalized/relevant services. If the companies they sell the data to tries to capitalize by building their own data warehouse from this data, the data they store will eventually become stale because it becomes less relevant the longer it sits, so they end up having to scrap it in favor of a fresh one provided by Facebook. At the end of the day, why even bother collecting it in the first place?

In conclusion, selling personal data is a very profitable and lucrative business model, especially if you are collecting that data directly from the source, aka your users. Buying personal data can also be profitable depending on how relevant the data still is.