Handwashing is not 100% effective either.
In combination with hand washing, social distancing, and general hygiene, face masks help further reduce the risk of infecting oneself, and inadvertently infecting others (if one is an asymptomatic carrier).
The evidence is in countries that have social norms for wearing masks (as opposed to the USA which promotes the social norm NOT to wear a mask). Hong Kong, for example, has very low infection rates despite being right next to mainland China, and a porous border where numerous Chinese enter it daily. Taiwan, Japan, and even Korea, have kept the infections and deaths well under control. They also have ample experience fighting off SARS in 2003. All these places admonish people who don't wear masks. Some even have laws to make it mandatory to wear a mask.
That's more than enough real world evidence to show the efficacy of wearing masks.
If the authorities want to send the message that "wearing a mask is effective at protecting you, but we have a shortage, so you can decide to increase your risk while giving the masks to medical workers" then I am all in support of that statement. People should volunteer to offer up their own protection to protect others. This is altruism.
But it is disingenuous to give the impression to Americans that wearing a mask is just not effective. This is not altruism. It is deception.