If facebook was driven purely by the motivation to help people stay in touch with their friends and to find events going on it would be a truly wonderful platform. Virtually every issue on facebook comes from seeking profits. At least problems from facebooks side anyway. There is also the social issues of propaganda and jealousy but facebook would have more time to deal with these when they aren't making the company more money.

Assume this in every situation and you'll never be disappointed ;)

1. Awareness. I don't think people are aware of how/what services are collecting data and how that data can be collected

2. Influence. Its hard, I imagine, for a lot of people to drop social media altogether. Its not all vanity. My wife has a disability that sometimes leaves her bedridden for weeks. Without social media, she wouldn't be able to communicate with our friends unless they call/text/come over, which they do, but its not always feasible one of those things will happen, so following them on Instagram and chatting via Facebook Messenger is really helpful in keeping her spirits up in those times.

3. Inertia. I think a lot of the current outrage against Facebook has been media driven, in particular, I think after Trump got elected -

(just a side note here before I continue, I'm talking about a criticism of media in general, not democrat vs republican politics or anything of the sort)-

I have a strong feeling, that I can't really substantiate, so take it as you will, of course (I acknowledge I could be wrong), large main stream news outlets started digging around about the mechanics of that election, and stumbled into the Cambridge Analytica scandal as a result, increasingly their practices came under fire, in part because I think some large media organizations (rightly, in my opinion) blame their data harvesting practices on getting Trump elected in the first place.

This also brings up another point I find so sad: despite the openness of the internet, the mass media still reigns supreme in being able to influence the masses, and I (anecdotally) feel like the power of freely and ubiquitously available knowledge via the internet has not had the impact on this sort of thing that one would have hoped. It was one of the promises of the internet in the 90s, that we would all vastly become more informed and it would take vastly less effort (and it does, if you are looking for it).

Or why did hundreds of thousands of users actively choose to share their data with a random company called Cambridge Analytica?

The problem is not lack of choice. The problem is that people don't care.

They certainly have never chosen to do so. You can accuse them of participating in some innane quiz, but it was exactly the big scandal that not only the participants' data, but also that of their friends was sucked and resold to Cambridge Analytica without their knowledge by the "researchers".