https://www.vox.com/policy-and-politics/2020/3/27/21194402/coronavirus-masks-n95-respirators-personal-protective-equipment-ppe
The problem is about both supply and demand. Prior to the coronavirus outbreak, China made half the world’s face masks. When the outbreak took off there, China started to use its supply and hoard what remained. This problem has only spread since, as more and more countries hoard whatever medical supplies they can get — with some, like Germany, even banning most PPE exports. So as demand increased due to Covid-19 — not just from health care workers but from a general public increasingly scared of infection — there was less supply to go around.
Some public health officials and experts tried to tamp down on public demand by suggesting face masks wouldn’t help laypeople avoid infection. But experts told me this messaging backfired: There is evidence that masks help people avoid infection, and once the public saw doctors and nurses were using and wanted more of the masks, that likely fueled distrust toward what public officials and experts were saying — and people bought up masks anyway.