The implementation of Nix's build sandboxing is outside of the core focus of Nix. The macOS sandboxing code is famously buggy. Seems like a perfectly reasonable

The implementation of Nix's build sandboxing is outside of the core focus of Nix. The macOS sandboxing code is famously buggy. Seems like a perfectly reasonable thing to leave to a battle-hardened, standardized, outside implementation to me.