The value is that even given this, they're much more reliable than preprints, and much much more reliable than sensational Twitter threads or public forums. They manage to filter out a tremendous amount of crap, but a sufficiently crafty and dishonest author can still sometimes slip past -- as is possible in any system.
Scientists, especially in biology (possibly psychology too but i don't know), do look at papers as data points and not conclusions. The "conclusion-minded" treatment of papers is a sad artefact of the publish-or-perish system (have to add a ton of spin to your results to make them publishable) and of pop science journalism.
There is a whole field of science dedicated to the statistical analysis of multiple studies in prior literature, e.g. just search for _Meta-analysis_ in the context of medical research.
A state-of-the-art Meta-Analysis is indeed the strongest level of scientific evidence you can have to advise a healthcare decisions or guidelines.
The only purpose of these journals with what they call a high 'impact factor', is that they typically have their peer reviews performed by people who are regarded as the _best_ in the their particular field. Nevertheless, to assume that an expert cannot be conned is also naive.
