Actually, they often are. But Wikipedia articles are written by dozens or even hundreds of people who spent a significant part of their lives researching the topic. Individually none of those people can match that Britannica expert's experience, but collectively they dwarf him.
Britannica's expert model also risks some pretty biased and incomplete articles, depending on the expert chosen. There's a relationship between being prominent in a field and being willing and able to write a comprehensive, neutral overview of the field, but it's not quite the same thing, and some top researchers are very bad at the latter.