If you expect an encyclopedia to be an exhaustive record of all knowledge, then yes, it's never going to explain every subject. If you expect it to be a review

If you expect an encyclopedia to be an exhaustive record of all knowledge, then yes, it's never going to explain every subject. If you expect it to be a review of subjects, like an abstract -- as encyclopedias generally were before the advent of Wikipedia -- then it seems like something can be said on most notable topics. It's a difficult task, it not only requires a thorough understanding of a subject but a special skill as an educator. It's a hard task. Yes, it's going to be impossible for some articles, but if they're notable aspects of mathematics then it's worth a try. I note that shelves are filled with books explaining complex topics at lower levels (easier in a book than a few paragraphs!?). Also, YouTube is replete with explanations that get closer to being encyclopedic than simply splurging graduate level textbook pages into a wiki (which is also a useful thing, but useful to different people).