I’d guess the simple answer may be that people don’t care/know, but for those that have ditched Facebook for whatever reason, yet still use Instagram, it’s a little surprising.
Is it partly a result of it being a more focused/limited form of social media that filters out some of the stuff people didn’t like from Facebook?
Because it got popular when it was it’s own independent company, and THEN Facebook bought it.
And that purchase happened in 2012, well before Cambridge Anylitica broke as a story.
It’s more that people saw no reason to leave, and by the time there were good reasons, Instagram was too large and established to easily dump for most people.