Some of Wikipedia's biomedical citation policies can seem confusing to subject matter experts. The crucial consideration is context. Wikipedia is the encyclopedia anyone can edit. That also means it must be an encyclopedia that "anyone" can verify.

Primary literature is written by scientists, for scientists. It's not intended to be:

• taken as health advice by everyday people
• taken out of context and applied
• verified by laypeople

Anyone familiar with the scientific process also knows that huge swathes of research are disproven or ignored by other scientists. These studies aren't marked clearly for Wikipedia editors to ignore. That kind of work is left to literature reviews and systematic reviews. That's why Wikipedians favor them as sources over even the best primary source author in a top research journal.