There can't be evidence for something for which no evidence was sought. Coming from academic physics background, the state of medical research boggles me. Let'

There can't be evidence for something for which no evidence was sought. Coming from academic physics background, the state of medical research boggles me. Let's start from so called "meta studies". Why are they needed? You'd think that if the individual studies were done carefully enough, a single study would be enough to establish a fact. Instead, you see meta studies with sentences like "we examined 142 studies on X for Y, out of which 76% showed statistically significant improvement". If we assume that the majority "vote" here is the correct result, what the hell did the 24% of the studies do wrong? Why aren't the people who made them expunged from the community for doing questionable research?