> needed to fetch each dependency’s source code just to read its go.mod file and resolve transitive dependencies.

Python used to have this problem as well (technically still does, but a large majority of things are available as a wheel and PyPI generally publishes a separate .metadata file for those wheels), but at least it was only a question of downloading and unpacking an archive file, not cloning an entire repo. Sheesh.

Why would Go need to do that, though? Isn't the go.mod file in a specific place relative to the package root in the repo?

Go's lock files arrived at around the same time as the proxy, before then you didn't have transitive dependencies pre baked.