Why should you care ? Because maybe your data (with mistakes in it, too) will be used to make decisions about things you want (jobs, insurance), without you even knowing why you got denied. Because often that collected data gets exposed in a breach, and then scammers or thieves can use it for their purposes. Because letting your data get collected exposes the activities of your friends and family too, without their consent. Because if the safe majority of us allow the collecting to continue, the data of the threatened minorities also gets collected, and may be used against them in ways we don't like or expect.
The battle for privacy is not lost. It's not one battle; it's a long-running social policy issue, and it's an arms-race. We get new laws such as GDPR and USA's COPPA and California's CPPA. We get new technical tools, or changes such as Apple killing the advertiser ID in their phones.
Lots of your past data has been collected. But you're creating new private data every day: your location, activities, etc. Try to protect that data better. And you can reach back and try to obfuscate old data that's out there, by overwhelming it with new data. Or make it irrelevant, by changing phone number, email address, physical living location, etc. The fight is not over, or hopeless.
What's the upside ? Well, shouldn't it be your right to control your public image and your data ? If you freely choose to expose all your data, fine. But it should be YOUR choice. The upside is more control over your life. Less exposure to thieves and scammers. Less risk for people who are unpopular minorities.