1. While the data is valuable, you still need a medium for your ads. One without the other is way less valuable. If you had user data for 50M users, today, how would you monetize it? Facebook had the News Feed and Instagram giving you a stage to operate on that data.
2. Following 1.), a lot of that "stolen" data was used to buy more effective FB Ads. Cambridge Analytica didn't "steal" the user data to sell to blackhats. They used it to craft messaging on Facebook.
All in all, user data is relatively worthless to most advertisers unless its actionable. Even if facebook gave you the data, the fastest way of monetizing it was to buy more facebook ads. It's relatively more difficult to extract user data from FB and then use that data to buy Twitter ads.