Thank you for reporting a bug to Unity.
After reviewing the behavior, we've confirmed it aligns with the current design and intended use of the feature. We understand this may differ from your expectations or workflow.
Unity's core Editor intentionally doesn't keep a map of extensions to their packages, as that would be an ongoing maintenance burden across many optional packages. To import glTF/glb files, please install glTFast via the Package Manager.
We will close this case as 'As Designed.' If you have feedback on how the feature could better meet your needs, please let us know - we value your input and consider it in future improvements.