And indeed you can sympathise with them that it would be hard to perform such an experiment.
But the whole point of science is that it allows us to make predictions based on theories. And the current scientific consensus is that wearing masks is beneficial.
"Most studies were underpowered because of limited sample size, and some studies also reported suboptimal adherence in the face mask group"
https://wwwnc.cdc.gov/eid/article/26/5/19-0994_article
And indeed you can sympathise with them that it would be hard to perform such an experiment.
But the whole point of science is that it allows us to make predictions based on theories. And the current scientific consensus is that wearing masks is beneficial.