The argument is usually that it has limited effect when it comes to protecting the wearer.
Simpler masks or home made masks can probably prevent transmission somewhat from the wearer by reducing the reach of droplets.
The argument against masks is based on 1) that people want them to protect themselves 2) that there aren’t nearly enough masks to do so.
So as I understand it, the authorities that say “don’t wear a mask” isn’t answering the question “would transmission be reduced if everyone wore a mask?”, but rather the question “Should I get a mask to protect myself”. Those are two very different questions.
I haven't seen any evidence supporting the argument that they do not protect the wearer. The argument seems to be that they aren't 100% effective at that, and therefore shouldn't be used for those purposes, which doesn't make any sense. Even 20% reduction in the amount of incoming droplets is significant - both for protecting the wearer, and for reducing R0.
Part of the reasoning is that people who believe masks provide them full protection will take more risks than otherwise. Combine that with hoarding and other behaviors and it's possible that promoting widespread use of masks would cause new problems the west doesn't already have.
If we had successfully promoted "wear a mask when you're sick" as general US policy in advance we probably wouldn't have these supply chain issues and it would be safe to advise people just wear a mask at all times during the crisis. We didn't, so most people don't have any masks, which means they're going to look at this crisis + advisory and panic buy a big pile of them when supplies are already limited.
I agree that public health authorities have not lied, but that's not the point. "Would transmission be reduced if everyone wore a mask?" is exactly the kind of question public health authorities should be asking! The answer seems to be "yes" even for homemade masks. So, public health authorities should be promoting massive production, testing, and use of masks.
The argument against masks is based on 1) that people want them to protect themselves 2) that there aren’t nearly enough masks to do so.
So as I understand it, the authorities that say “don’t wear a mask” isn’t answering the question “would transmission be reduced if everyone wore a mask?”, but rather the question “Should I get a mask to protect myself”. Those are two very different questions.