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 use Instagram on occasion because it’s something almost everyone my age has and when your meeting new people, you often have to meet them where they’re at (digitally). I barely touch Facebook anymore because the interface is cluttered and half the features broke and were never fixed , like searching for events. I tried Pixelfed, but after a month, only one person has liked a single photo I posted. The app itself is great.
The reason I prefer Instagram over Facebook is universality in my age group and, as you pointed out, it cuts a bunch of the crud from FB and I actually see pictures of my friends rather than a bunch of stupid disguised ads and political BS.