It's about consent. A user gives explicit consent for a company like Instagram to have access to data. They don't give it to random third parties to scrape.

And no, they can't sell it to whoever they want (or at least that's a reduction of how Facebook and Google use your data). They do monetize the data with their internal advertising platforms that they sell to people like OP, rather than letting anyone monetize the data without the knowledge or consent of their users.