> The increase (in the number of meta-analyses) is a consequence of the higher prestige that systematic reviews and meta-analyses have acquired over the years, since they are (justifiably) considered to represent the highest level of evidence
No. The increase is because it is cheaper to do meta-analysis that it is to design and conduct experiments. They also carry less reputational risk.
1. They make excellent student projects. Part of this is cheapness, sure, but part of it is that meta-analysis can be done relatively quickly. Some observational studies will take years to complete - in the meantime, your Masters student needs something to do.
2. They are often "Step 1" of a number of study designs. For example, if one is eliciting priors for a Bayesian analysis, or in my case trying to parameterize a theoretical model, "Is there a meta-analysis on this, and if not, can we do one?" is one of the first questions asked.
3. It allows participation in a field. For example, I have thoughts about some aspects of clinical medicine. I am unlikely to ever run a clinical trial, what with not having a position in a medical school. I can however perform a meta-analysis of trial data as well (or possibly better) than the people performing the studies. Running a study and conducting a meta-analysis are not necessarily the same skill set.
There's an undertone in this thread that meta-analysis is just what you do if you can't run a study. I wanted to note that there are scientific reasons to perform one in addition to logistic ones.
No. The increase is because it is cheaper to do meta-analysis that it is to design and conduct experiments. They also carry less reputational risk.