The classic correlation does not mean causation fallacy
In the absence of putting some states in a double blind experiment where they only get placebo procedures, which would be immoral if one thinks that might harm them, then correlation is all we have.
And well.. there are indeed plenty of double blind experiments that have been indeed done on animals and in laboratories to test hypotheses on transmission.
But when there is a high correlation, and you have sound scientific theories that have indeed been tested that explain the correlation, then you act with the best information you have.
Or... you can play the part of an epistemological skeptic just so you can ignore data that might cause you to have to deal with the discomfort of wearing a mask.