[neonsunset]

Domains that require network-isolated builds usually maintain internal mirrors with corresponding dependencies. It is unreasonable to expect from a project of this size to have all tooling it depends on to be available within repository files (moreover, it should build on multiple ISAs and OSes). I doubt you can build LLVM this way or, let's say, OpenJDK.

[gray_-_wolf]

Well for example golang builds offline just fine, which seems similar in scope? Runtime-based language targeting multiple architectures and operating systems.