Women didn't think they needed to hide their periods from the government. Until the rules changed and suddenly period tracking apps become a vector for lawsuits against women.
You don't always know what you're doing now that seems perfectly normal that will suddenly be used against you in the future. Or which data will mark you as a target.
The point is what one wants or needs to keep private can change day to day. Tracking womens' menstrual cycles was a joke in The Office under Roe v. Wade. Today, it's much more serious.
That's because we are so weak as a nation we can't come together and say "fuck that" to companies owning any bit of data they can collect about us without permission. Could be solved by laws, but we are far too divided for that.
Regulations exist around handling this kind of data. As long as you have privacy of medical decisions, the government or third parties shouldn't be able to access them.
I'm not sure why you're being downvoted. What you are saying is correct.
Now yes, society can make laws about private information to reduce the spread, like GDPR, but fundamentally that just stops the data spreading (a bit). The root data is no longer "private" - it's just shared with fewer people.
There are compelling reasons to share data. Whether or not it's enough to compel you isn't really the point.
Most companies that are harvesting data are using it to offer some sort of service to the users' benefit, on top of using it to advertise to them. Sometimes the benefit is just "it syncs across devices and I don't have to run my own server." Not everyone is technical enough to do that.
And sometimes, things just see very straightforward, and not worth hiding. Different people have different levels of tolerance for things. For some people, period tracking is very much health information. Some may just find it very personal and view it as such and not use an app, or some may be trying to get pregnant, so it's explicitly health related but worth the risk... but for others, is just something you do, and it's not "health information," it's just trying to keep things predictable.
Everything comes down to "if things change, what are the odds this could be used against me?" when it comes to privacy. Maybe talking about privacy online becomes a negative thing under a totalitarian government that wants to quash that kind of speech, and this conversation we're having right now is a terrible idea. Probably not, but just sharing your opinions on things (literally any topic) is private information that could used against you, if you construe it properly.
But we consider that scenario unlikely, so we have the conversation and share our thoughts and express ourselves online. And a little bit of data about us is added to the pool.
The legislature tried to pretend they weren't criminalizing abortion, by declaring that totally uninvolved randos can sue anybody for having or somehow helping in an abortion, and take thousands from the defendant(s) as a reward.
It's crazy because the usual rule is that you can only sue for harm that happened to you, but Karen McBusybody stalking your Facebook feed wasn't hurt, she just wants to strike out at "evil harlots" while making $10,000.
You don't always know what you're doing now that seems perfectly normal that will suddenly be used against you in the future. Or which data will mark you as a target.