"Barely any reason"... except they created and maintain the entire plarform and tooling that you're building on. And in Apple's case they give it away for free with any mac.
I'm old enough to remember when buying development tooling for DOS or Windows was $$$$$$
Apple started giving away the development environment because they had such an anemic software ecosystem. They had a handful of OpenSTEP developers and a larger crowd of die-hard Mac people, the successful ones mostly moving away from the platform.
Today Apple is taking percentage of every dollar made from application developers who participate in their App store and they are making it increasingly difficult to avoid this with every release. IMHO, they are making far more dollars today than they ever did selling development hardware and SDK licenses.
Both of these are completely false. Testflight distribution without a developer license is impossible. Asking users to compile the app themself is infeasible, as the XCode toolchain is upwards of 18gb and they will be required to compile it once every week to keep it on their device. The developer fee is unavoidable — even with EU intervention
I'm old enough to remember when buying development tooling for DOS or Windows was $$$$$$