The year following our graduation, my classmate heard from the tutor who runs the related course -- asking why the hell his students were claiming he invented such-and-such technique! Several students had cited Wikipedia without checking sources.
To answer your question, Wikipedia is a lower quality source than scientific journals for the same reason direct democracy isn't usually as good as representative democracy. We delegate trust in matters to an authority. Sometimes there are problems with the quality of the authority but at least there is a framework to work within.
Also, most Wikipedia articles worth citing contain really bad factual errors. Since most readers aren't topic experts they don't notice such errors. But they are there and they are really bad. I only have some math education but I've found lots of mistakes in proofs on Wikipedia. Imagine how bad it is for higher-level math that is beyond the reach of most visitors to the site.
That’s the part that matters.
But most WP users are quite passive, and unfortunately a very few are extraordinarily active, so the quality of the WP they patrol, depends heavily of their biases. As they are few, there is no natural moderation.
But having a quality article is not enough, it is also important to have quality articles on a reasonable range of topics, and this is not true for some WP (French WP for example)
Same article since 2012 about cancer vaccines, only 42 words! https://fr.wikipedia.org/wiki/Vaccin_contre_le_cancer