On the contrary, the problem with undergrad math articles is they are usually written at the wrong level. They are often written at a graduate level, where the function of an introductory article is to introduce someone to the topic. You should be able to understand any encyclopedia article just with a high school education imo (its an encyclopedia, not a scientific paper or a textbook), or at the most you should maybe have to read 1 or 2 other articles for background.
I did a double-check on a few "simple" math topics, and -- yeah -- I think you're right. The Pythagorean theorem[1] article (even though rated as "Good") is all over the place, not to mention way too long. If I want a super simple introduction, Math Planet does a much better job[2]. And if I want a more technical deep-dive, Wolfram Alpha does a better job[3].
