No one knows why the CDC issued the guidelines that they did. There's also fairly little doubt that the CDC in previous decades would've been smarter and better organized.

A lot of the commentary about the matter is disingenuous. Of course they won't filter out all the infectious agents for all healthcare workers forever. It was a matter of risk mitigation, just as much as shelter-in-place is. Getting infected is a numbers game, as is the live-locking of available hospital capacity.