Evidence is nice, but sometimes it makes sense to reason from known principles. SARS-CoV-2 was known to spread through respiratory droplets almost from the beginning. Particulate respirators are known to protect people against respiratory droplets. Surgical masks are routinely used for source control of respiratory droplets in surgical settings. A mask recommendation made a lot of sense, even before there was data suggesting they were specifically helpful against SARS-CoV-2.
"For respiratory exhalation flows, the critical size of large droplets was also between 60 and 100 μm, depending on the exhalation air velocity and relative humidity of the ambient air. Expelled large droplets were carried ... less than 1 m away at a velocity of 1 m/s (breathing)." --
https://onlinelibrary.wiley.com/doi/full/10.1111/j.1600-0668...
Hence the idea of social distancing (for airborne droplets) and frequent hand washing (for surface contamination). (And, of course, staying home if you have symptoms like coughing or sneezing.) Convincing people to use masks consistently and correctly has previously been shown (and remains!) to be an uphill fight.
(I note that there was a meeting of epidemiologists in March, 2020, where famously, no one wore masks. How far are you willing to to go to lie?)
Later, it was found that viruses could be carried by much smaller particles, much farther.
"Our laser light scattering method not only provides real-time visual evidence for speech droplet emission, but also assesses their airborne lifetime. This direct visualization demonstrates how normal speech generates airborne droplets that can remain suspended for tens of minutes or longer and are eminently capable of transmitting disease in confined spaces." -- https://www.pnas.org/content/117/22/11875
