Why? If some open source maintainer goes off the rails and deletes all their packages, why should that break my builds? I still have a valid license to the code, I don't really care that a maintainer rage quit 4 dependencies down from my application. I certainly don't want to have to scramble to deal with that.
Do they change the behavior if the repo was dropped for legal issues, and no valid licenses could have been obtained because the repo owner didn't have one in the first place?