Mostly because if I don't like one book by an author, I'm not too likely to read his or her other books, and having read the one book doesn't really make me feel qualified to express an opinion on the worth of the author in general.

For instance, I could say that I was really not a fan of Stendhal's "Le Rouge et le Noir", or, in a completely different generation, Peter Carey's "The Chemistry of Tears", and sure, my impression of either author was not improved by the experience. But not having read any other novels by either author, I wouldn't dare to say that they are overrated.

And honestly, in the vast majority of cases in which I've read an author who's supposed to be good - claimed to be good by people whose opinion/taste I have some confidence in - my conclusion has been that the book in question was indeed good. Or at least that the author was good - when I read Atwood's "The Handmaid's Tale", I found it a hatchet job in poor taste about which I can't say much more positive than that it's competently written, but my feeling that I might still like other books by her was proven correct afterwards - when she's writing novels instead of bizarre propaganda, she's a very good writer.

The only major author about whom I occasionally think that she might be overrated, while actually having read enough of her work to have a valid opinion (all of it, even) is ironically one whom I really enjoy reading and do really appreciate, Jane Austen. I love reading her books, and I'm not surprised that tons of people do; what does surprise me is how people will rank her among the all-time greatest English writers. That, I do not really see. Perhaps it's because I'm not quite familiar enough with the field to properly appreciate the status of the English novel before her, and which innovations she may have brought to it; the authors I'm inclined to compare her to are, in fairness, all writing at least several decades later.

I think I'm just not a very critical person that way - I'd be hard-pressed to give you much of a list of overrated people or things in most any regard, and even when I do think of something as seriously overrated (Metallica comes to mind as an example), part of me still thinks that I haven't exercised enough due diligence to make that call. And even in cases of the opposite situation - that I'm reading a book, watching a movie, etc. that most people with good taste find bad - I'm more likely to play devil's advocate and look at the positive sides of the work in question, if any can be found at all.

That's a lot of words for a post that really doesn't answer your question, isn't it?