If you manage to do double-blind studies for every single piece of knowledge out there, kudos for you. There's nothing bad in this.

Anyway, it's on topic for several sidelines people are raising. But not on topic for the main article.

I think the real point is that systemic reviews often will have a pretty tilted set of included studies, because they are influenced by what things researchers choose to study.

Indeed, you probably couldn't publish a study saying that parachutes work; it's not an interesting enough finding for publication. So the only stuff you'll find, in many cases, are studies that buck the prevailing wisdom.