Greed. Same reason people play the lottery. Because they think there's an easy route to money and success. That may sound cynical, but that's the hard truth.
I agree fully with Scott and the others that it's important to educate buyers. But--frankly--I suspect that the large majority of Realtors do. Or try to. But the seller's thinking, "I can buy a house that sold for $400,000 a few years ago for just $175,000. I'll let Debbie Downer prattle on a bit about all the hassles, but my mind's made up. I'm not going to pass up this deal!"
And we can mix in an ounce or two of dumbness, too. That applies to every buyer who refers to a short sale as a "quick sale." Or, after having it explained to them that the lender has to approve the price, are shocked that their "full price" offer didn't succeed.