The answer to the question of why THAT is the case - is not so easy to answer. The main benefit I can see with using lock files instead of vendoring is that it saves a lot of storage and diff history from entering your repository. So clones are much faster, backups smaller etc.
I think Go used to work this way (automated vendoring) but it’s the only language I can think of that ever did in terms of standard tooling. It would be instructive to learn why that changed.
I use dotnet and I never liked seeing dlls and binary files in my diffs. I would argue if we are adding vendor code to our projects, we should demand the FULL source code instead of dlls. Maybe it is already possible with things like x unit. I have never given it much thought... But then that vendoree code has to come from somewhere as well, right? I mean there is something to be said about provenance or something here?
Sorry if this feels like a stream of consciousness because it is ↔
It bloats your repo, both with the actual code, and the large diffs when you update it.
You have to manually track new versions, without something to tell you if new versions are available, or if your version has known security vulnerabilities.
If the dependency has it's own dependencies, you have to vendor those too recursively. And if multiple dependencies have the same transitive dependency, it is up to you to deduplicate them, and make sure you have a version compatible with all dependents.
Etc.