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’ll add that you can buy a $30 used “thin client” pc via eBay and set up Ubuntu server (or your choice of Linux distro) then set up pi-hole on it. Cheap project, lots of value