There's nothing stopping people except the time and effort required. For fontconfig and some other nice to have programs (X itself builds fine) you need to eith

There's nothing stopping people except the time and effort required. For fontconfig and some other nice to have programs (X itself builds fine) you need to either patch them or modify the SDK you're building against to remove the compiler attributes. Apple won't allow the latter on the app store, and the former would be a significant amount of work. There are terminal emulators and X servers on the app store now, but the problem is the limitations of the sandbox and performance. Alpine and an X server can be run on Qemu, but performance is far better with the native server. And, Swift Playgrounds is an Apple product. They don't hold themselves to the same limitations as 3rd party app developers.