> It is a critique (in poor taste if you ask me) that everything does not require a double-blind study.

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.