Please correct me if am wrong since I have never made any mobile OS applications.
There’s the $100 fee or whatever to join the developer program even if your app is free. There is a slow manual review process. You have to share a 30% cut with Apple of any profit. Most of the tooling for iOS requires a macOS machine which is another massive money sink as well as being proprietary with no source available. I believe GPL or similarly licensed code isn’t allowed on the App Store either due to ToS conflicts.
Most privacy-friendly things take an source available, open source, free software, or other (post-open, copyfarleft) approach that conflicts with Apple’s model for software, but also developers are likely to be on Linux or BSD + a Linux or Android phone where everything will be more arduous to build & test while not even being a device they or their friends would use so why both building if likely you aren’t even getting paid for it.