Vendoring isn't a clear win for security. It may provide some protection against "supply chain" attacks, but makes it more difficult for you to respond to vulnerabilities discovered in the version you vendored. And for business continuity, in many cases depending on an external registry is an acceptable risk.

And both of those concerns can be addressed by using an internal/private registry that mirrors the packages you need.

But in any event, your original question was to elaborate on why vendoring is inconvenient. Whether or not the benefits are worth the inconvenience is a different question, to which IMHO the answer is "it depends". Sometimes it is, and sometimes it isn't.