This also resonates with recent sentiment that telling people that "masks don't work" (with the intention of keeping people from buying masks when they are scarce even for health care workers) can backfire significantly:
https://www.nytimes.com/2020/03/17/opinion/coronavirus-face-...
The major reason why masks are not promoted on a wide scale is because there simply are not enough masks right now. Simple DIY masks are not perfect, but certainly better than the current state of hardly any use of masks in public at all.