It's not. But it's rather trivial to run your own registry, and many (most?) companies do exactly that.

> It's up to you to make sure that what you add to your project doesn't contain malware/vulnerabilities.

That's why, again, many companies run their own registries, and don't download random files from the internet.

> Having a lockfile will ensure that if you download a dependency to review it for vulnerabilities, later re-downloads of the dependency will not have changed files.

And to generate that lockfile... you need to first download a random file from the internet. Got you.