There are different, less self-interested kinds of emotional investment, though. Many article authors are intellectually interested in their subjects, but not personally connected to them. For example, there's a difference between writing a Wikipedia article on a mathematical theorem you consider beautiful and important; and writing a Wikipedia article on a mathematical theorem that you personally discovered and published. In both cases you care a lot about the subject, but in the second case it may be better to let a third party who's less personally close to it write the article.

(Interestingly, this almost never comes up in the mathematics articles. There are quite a few mathematicians who edit Wikipedia, and they tend to be quite scrupulous about that kind of thing.)