> What about the .NET build process makes vendoring the only option?

AFAIK, even NuGet doesn't allow you to just check in the dependency spec, rather than the contents of all of your dependencies.

> Even so, it's always been more convenient in my experience when something just ships with fixed versions of dependencies already inside it.

I agree. But that doesn't mean that they belong in Git, nor that they should even be copied to every single project.