--fork would prevent zig-pkg directory from being created, because it takes effect before fetching.
For example, say your project has only 1 dependency, bar. You don’t have bar fetched yet. But you do have a source checkout in /home/you/bar which happens to be the correct version, or perhaps newer but still compatible. You don’t have Internet connectivity right now, so you use zig build --fork=/home/you/bar. In this case, zig-pkg directory is not created because all dependencies are already fetched or overridden.