[jacobolus]

The key words here are can be. Some mathematics articles are high quality, but most are somewhere between completely inadequate and blandly mediocre. If you click through all of the math-related wiki-links in even the best mathematical articles, at least half of them are going to be a disappointment. Many articles are little more than a dry definition. In general articles are missing context, history, proofs, figures, references, .... Topics that dozens of top mathematicians focused decades-long careers on, for which there are multiple hundreds-of-pages-long excellent textbooks and dozens of papers describing the historical development, have a few measly paragraphs of text and no figures. Some high-level articles contain little more than a bullet list of wiki links. Biographies of mathematicians (except for a few exceptional famous people) generally have little to no mathematical content, and do not put work in context. Mathematicians writing in English (and some older ones in German/French) get coverage but there are huge gaps in biographies of e.g. Soviet mathematicians.

The best coverage is probably of the content of typical secondary school / undergraduate math curriculum. But even then articles are often full of lists of trivia while neglecting motivation and central ideas.

The same is true for many other parts of Wikipedia though. Writing good Wikipedia articles takes a ton of work (research, organizing, writing, revising, drawing figures, building consensus with other editors, ...), and is a distributed volunteer effort, so the quality of material that does make it online is still surprisingly impressive.

[kragen]

If we can get those top mathematicians to license their textbooks and papers under CC0, CC-BY, or CC-BY-SA, we can incorporate their explanations and figures into Wikipedia.