They don't. A case went to the Supreme Court and said Congress needed to make that decision, not an unelected CDC, but that they wouldn't rescind the current existing order...which then expired. Biden basically said as much, and that he was going to do it anyway and extended it after it lapsed. For some reason, the news doesn't really cover that Biden is in direct violation of the Supreme Court with his latest extension and he knew, publicly said so, and did it anyway.
> Biden said on Tuesday he had sought out constitutional scholars to determine the CDC’s legal authority and “what could they do that is most likely to pass muster constitutionally.” The “bulk” of the scholarship reviewed by the White House said additional action by the CDC would not pass muster in the courts, Biden said, adding that there were some “key scholars” who said it could.
https://www.politico.com/news/2021/08/03/eviction-moratorium...
The bulk of constitutional "experts" said it wasn't constitutional. He found one or more that didn't agree with that assessment, so he went with the few against the majority.