You're only thinking of the federal level. There are many levels of government, and local governments from states and counties all the way down to companies contracted by the local dog catcher buy the data mined by advertising companies.
and has been for 30 years - you can buy lists for direct mail of people's name, full addresses, numbers, marriage status, estimated income, etc
But for 30 years you haven't been able to track someone's location on a minute-by-minute basis. Or in real-time in the case of buying data from a cell phone carrier. Plus a lot of other invasive information that hasn't been available until FB, G, and others weaponized the avertising industry.
We're not talking about public data. We're talking about companies selling private data.
Apart from the evil of "making people buy shit they wouldn't have otherwise" that applies to all advertising the end game is to do price discrimination based on profiles of the individual. If they know you're friends are raving about a holiday somewhere then you'll see a higher price than a casual browser. This way you can extract the maximum amount possible from your customers.
You are looking at it the wrong way. It's not about Facebook's evil end game today. It's about what their current collection of data enables a future Facebook (or a company that may buy Facebook one day) to do. It's also about what malicious actors, government or otherwise, may do with the data if they get their hands on it, as they have, multiple times, in the recent past.
Today the data is used to show you ads. There is no guarantee that it will be used for just that purpose in the future.
The government doesn't need Facebook to get your data (they go straight to the ISPs, anyway). There's 10,000 companies that buy and sell your personal data already (and has been for 30 years - you can buy lists for direct mail of people's name, full addresses, numbers, marriage status, estimated income, etc.)