I think you should look at the background papers before making such a bold and unwarranted dismissal. Any researcher worth their salt will make sure to at least try to exclude other factors, and any good peer reviewer and journal editor will point out if this is missing. So if papers consistently find this link chances are very good it's there.
Actually, this might be an example of the "healthy user" bias in observational studies.[1] Although epidemiologists do try hard to include all of the relevant variables in their analyses, there are likely unidentified individual differences (perhaps including personality and lifestyle) that distinguish people with better health outcomes.
That's why double-blind, randomized, controlled trials provide more compelling evidence of cause and effect.
