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?
I’ve come across people who didn’t know Instagram is owned by Facebook, and people who thought meta and Facebook were different companies. I thought the rename was the stupidest thing ever until I saw it did actually work on some people.