Because, let's be honest--what we do in, say, English departments is fundamentally similar to what fangirls do over IMs or at sleepovers. The only difference is that while they do it with Harry Potter, Jack Sparrow, Angel Sanctuary, etc. etc. etc. etc., we do it with Huck Finn, Emily Dickinson, Moby-Dick, etc. etc. etc. etc. (And for people who think that discussion of slash within a fandom is a serious waste of time, well, I recommend you spend some time with your local queer theorist.)
In other words, the academics do the same stuff as the fangirls. The only difference is the fandom. And this goes for the bad aspects of both activities as well as the good-- pretty much the only difference betweeen, say, Fandom Wank and The Papers of the Modern Language Association is that one uses "XD," "lol," and "omgwtfbbq!!!11one," while the other uses "hegemony," "essentialism," and "epistemological."
Six of one, half-dozen of the other.