They really aren't if you think about it. Going straight to a URL for a version of a dependency is the same as pulling it from a registry, except it's decentralized from a single source (NPM) and removes the extra hops in between the package vendor and the package consumer.
On the flip side, that extra hop adds a ton of convenience in the form of name-resolution, security and governance. It's the age old double-edged sword of centralization.
Maybe if you turn that url into a hash, than just use the hash to check if the package has a local copy already, it wont be so bad. But you will need to add the package version in the URL so that you know you will always have the package you really want in your local cache.
On the flip side, that extra hop adds a ton of convenience in the form of name-resolution, security and governance. It's the age old double-edged sword of centralization.