Apple's Xcode is a free download, the Swift compiler is open source. However, to become an "official" developer on the Apple platforms and publish apps you do need a paid developer account.
And if you're doing ruby or nodejs or a handful of other languages, you can use the XCode CLI and most of your code (possibly all) will never see the App Store submission queue.
It makes sense for Apple though, because what the devs are really paying for is access to Apple's closed-source ecosystem, not the tooling itself. This company has no such market power; why would anyone want to pay for Zen, when it has no established ecosystem or audience?