I‘m guessing you haven‘t used too many „real“ encyclopedias? The quality of writing on those was very high. In terms of readability, understandability, and reliability, I find the average print article to be significantly better than your average Wikipedia article. Their big downsides being, of course, that they were not as exhaustive on any given topic as Wikipedia is (though this actually helped with the understandability), and they were quickly out of date (especially as they were too expensive to replace every few years).
One of the things that traditional encyclopedias (as well as a given book/magazine/etc.) have is a very clear view of the persona that they're writing for. With Wikipedia, this differs considerably from article to article depending upon the topic. With respect to many technical topics, especially in areas like math and physics, Wikipedia articles often dive right into equations and many articles are largely incomprehensible to anyone not already familiar with the subject matter.
And on the flip side, there are other technical topics where large parts of the article are written by people unschooled in the subject citing magazine and news articles which are online rather than the field's core texts which aren't...
This could be considered more of a problem with scientific publishing (in those fields). This is changing though, so you might see wikipedia start to reflect that.
