Because that was the common belief at the time among medical professionals. Everyone I know who is a medical professional told me masks wouldn't help, I did my own research and it seemed logical a single 0.1 micron virus could get through an N95 and infect me, so I parrotted it as well. When I saw enough counter evidence, I changed my mind. And whatever efforts went into that CDC statement, 10x the efforts have gone into undoing it. The horse is dead. Like, really, really dead. Like, it's-already-been -sent-to-the-glue-factory dead