Because every license is eventually a piece of content; I assume it also needs a license; using a license to license itself would lead to circular logic - is that a problem in the content licensing world? If so, one solution is to ship any public license with just a mere custom legal notice
You are correct that the text of a copyright license is itself also protected by copyright.
Most copyright licenses do not acknowledge that fact and thus don't contain a license for the text of the copyright license.
A very strict reading of copyright law would mean that only the author of a copyright license would be allowed to reproduce the license text. But judges are not machines that literally apply the the law. They are humans who can also look at the circumstances. If a license text gives a recipient the right to reproduce and redistribute a work that is covered by the license, then the recipient of the covered work must by necessity also receive a license to reproduce the license text verbatim, even if that was not spelled out explicitly.
For those licenses that do acknowledge copyright protection of their text, they invariably have a rather restrictive license for the license text, ranging from only allowing verbatim copies to allowing you to re-use the license terms under the condition that you call the license something else.
Releasing the text of a license (open-source or not) under permissive terms is something that can cause great confusion because it will invariably result in multiple, widely different, licenses that go by the same name. The end result is that those licenses will be shunned by any project that takes itself serious.