If you read that page, it also explains that it also has a fallback to a very permissive copyright licence in cases where you cannot release a work to the publi

If you read that page, it also explains that it also has a fallback to a very permissive copyright licence in cases where you cannot release a work to the public domain. Not all countries have a concept of public domain, and many that do have arbitrary restrictions applied to it. In fact, this is one of the reasons you should use CC0 rather than just saying "public domain" or using something like the Unlicense.