> Also, any kind of software development outside of macOS is strictly forbidden")

This may have come to be an intentional moat, but I don't think it started as one. Remember that iOS and its derivatives are themselves OS X derivatives, and that there was little to no demand for OS X dev tools that ran on other platforms prior to iOS. It was originally more of a historical artifact of all of the tooling being deeply steeped in Apple's own tech stack.

Back in the Rhapsody days there was a version of Cocoa, Project Builder (Xcode predecessor), etc that ran on Windows, but that got scrapped likely because at that point Apple was not the juggernaut that the iPhone turned it into and it was likely judged too expensive to maintain for dubious benefit.