It could be in part that health authorities haven't recommended mask use en masse by the general public to prevent the possible asymptomatic spread of constantly changing annual flu strains because studies have not yet determined how dangerous or common or likely that is:
"Although asymptomatic individuals may shed influenza virus, studies have not determined if such people effectively transmit influenza....Based on the available literature, we found that there is scant, if any, evidence that asymptomatic or presymptomatic individuals play an important role in influenza transmission. As such, recent articles concerning pandemic planning, some using transmission modeling, may have overestimated the effect of presymptomatic or asymptomatic influenza transmission. More definitive transmission studies are sorely needed."
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646474/
"Symptoms can begin about 2 days (but can range from 1 to 4 days) after the virus enters the body. That means that you may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick. Some people can be infected with the flu virus but have no symptoms. During this time, those people may still spread the virus to others."
https://www.cdc.gov/flu/about/disease/spread.htm
https://virologydownunder.com/influ...or-without-symptoms-youre-dropping-flu-virus/
"As for the possible role of asymptomatic carriers in spreading flu, Bresee commented that symptoms correlate with the amount of virus shed. "So my presumption is that asymptomatic infected people will shed less virus than people who are symptomatic, and therefore will contribute less to community spread," he said. "But I'm not sure how much data there are for that."
https://www.cidrap.umn.edu/news-perspective/2014/03/uk-flu-study-many-are-infected-few-are-sick
"The presence of influenza viral shedding in patients with influenza who have very few or no symptoms reflects their potential for transmitting the virus to close contacts. These findings suggest that further research is needed to investigate the contribution of persons with asymptomatic or clinically mild influenza virus infections to influenza virus transmission in household, institutional, and community settings."
https://academic.oup.com/cid/article/64/6/736/2733100
"A commentator says an important unanswered question is how much the asymptomatic cases contribute to flu transmission."
https://www.jwatch.org/fw108600/2014/03/17/most-flu-asymptomatic
See above.
Also, BTW, mask wearing was recommended during the Swine Flu outbreak in 2009 & masks were commonly worn at the time of the flu pandemic of 1918 in America & elsewhere:
"Swine Flu (H1N1) and Face Masks
When and how to use face masks in home and community settings."
https://www.webmd.com/cold-and-flu/features/swine-flu-h1n1-and-face-masks#1
https://www.webmd.com/cold-and-flu/features/swine-flu-h1n1-and-face-masks#2
https://www.cdc.gov/h1n1flu/masks.htm
Note also the present recommendations re the annual flu:
"Avoid crowds. The flu spreads easily wherever people gather...Your local health department and the CDC may suggest other precautions to reduce your risk of COVID-19 or the flu. For example, you may need to practice social distancing (physical distancing) and stay at least 6 feet (2 meters) from others outside your household. You may also need to wear a cloth face mask when around people outside your household."
https://www.mayoclinic.org/diseases...al infection,that cause diarrhea and vomiting.
https://www.cdc.gov/flu/symptoms/symptoms.htm
See above. Actually during the 1918 flu pandemic healthy people wore masks. They are also often worn in more recent years by healthy people in hospitals and other medical settings, etc.
Mask wearing seems like a small price to pay to save many millions of lives in the future as well as spare many millions more from illness, hospitalization, loss of work, ICU, long term negative health effects, etc. Many people such as myself would only need to wear a mask for about 30 minutes a day.
The vast majority of health experts & countries are mandating or recommending mask use & are quite aware of the studies on the matter.