Obviously popular articles are great -- they have so many eyeballs and editors that they're not just quite accurate, but often more comprehensive than other sources (in terms of describing competing schools of thought, for example).
But when you really drill down into more niche articles, there's a tremendous amount of information that is uncited or not found in the citation, has glaring omissions, and/or is just plain wrong. These are the kinds of articles that get 1 edit every six months.
It's those latter articles that are the reason Wikipedia is too unreliable to cite.
(Also, Wikipedia is a tertiary source, as it is meant to only cite secondary sources, not primary sources.)