Anyone concerned about their privacy wouldn't use these services to begin with. The fact they are so popular is indicative that most people value the service over their privacy, or simply don't care.
Yes, they want to use everyone's data. But they also want everyone as a customer, and they can't have both at once. Offering people an opt-out is a popular middle-ground because the vast majority of people don't care about it, and those that do care are appeased
They have every incentive not to, and no oversight to hold them accountable if they don't. Do you really want to trust your data is safe based on a pinky promise from a company?
Or, the general populace just doesn't understand the actual implications. The HN crowd can be guilty of severely overestimating the average person's tech literacy, and especially their understanding of privacy policies and ToS. Many may think they are OK with it, but I'd argue it's because they don't understand the potential real-world consequences of such privacy violations.
That might've been the case in the first generations of ad-supported business models on the web. But after two decades, even non-technical users have understood the implications of "free" services.
IME talking to non-technical people about this topic, I can't remember the last time someone mentioned not being aware of the ToS and privacy policies they agree to, even if they likely hadn't read the legalese. Whereas the most common excuses I've heard are "I have nothing to hide", and "I don't use it often".
So I think you're underestimating the average person's tech literacy. I'm sure people who who still don't understand the implications exist, but they're in the minority.