I think the problem is that each study has to make many arbitrary decisions about aspects of the experimental protocol. This decision will be made the same way for each subject in a single study, but will vary across studies. There are so many such decisions that, if the meta-analysis were to include them as dependent variables, each study would introduce enough new variables to cancel out the statistical power gain of introducing that study.
I think the problem is that each study has to make many arbitrary decisions about aspects of the experimental protocol. This decision will be made the same way for each subject in a single study, but will vary across studies. There are so many such decisions that, if the meta-analysis were to include them as dependent variables, each study would introduce enough new variables to cancel out the statistical power gain of introducing that study.