Systematic reviews like those done by Cochrane search the literature for relevant studies then apply pre-set standards to exclude those that have a high risk of

Systematic reviews like those done by Cochrane search the literature for relevant studies then apply pre-set standards to exclude those that have a high risk of bias. That's why so few studies were included. It's not their fault that good studies haven't been done. The answer is to do sufficiently powered RCTs to resolve the question, not conclude that masking works on the basis of low quality observational studies.