Because there needs to exist some bindings to the iOS framework. This is the most complete bindgen of the docs as it uses all of the objective-c features in rust-bindgen.
cargo doc --open --target aarch64-apple-ios and then read the docs.
Admittedly, it would be nice if the docs had links to the original apple docs
but that's actually kind of hard to do.
Here are the docs: simlay.github.io/uikit-sys
Maybe when I get the time.