EDIT (adding an appropriate - though ancient - quote and reference):
"Internet commerce appeals deceptives (sic) to a particularly male fantasy. Guys like the idea that after a short initial period of programming, a computer will tirelessly slave away for them, making them money 24 hours a day. Set up the site, walk away, and watch the money pile up in your bank account.
You can feed this fantasy by reading articles in the business press about http://amazon.com, the perennial poster children for Internet commerce. They set up what is essentially a front-end to a wholesale book distributor's database, and now they are selling books every few seconds. It sounds like they are rolling in money.
Well, it turns out that I know some people who work at amazon.com. The customers don't always fill out the forms exactly right. The books aren't always in stock like they should be. The customers send e-mail asking when their books are going to be shipped. So instead of one Unix box and a big vault for the cash, the company has 200 employees sucking all the money out of it. And remember, this is the best that anyone has really done: high expenses and high sales. More typical is an Internet store with high expenses and low or no sales."
They're just slightly above 419 scammers.
I've been involved with a friend's site that pretty well fits the category. After some initial programming, SEO, etc, it did very well for 2+ years (> $100k revenue), but user support grew and grew, and then clones popped up, and started ranking ahead of him in Google results, then the revenue dropped to next to nothing. He could invest more time to get back to the revenue level he was at, but I just think that's the natural cycle for "passive" web sites.
Things go wrong, competition appears, users never quite fit the mold you want them to fit in, bad things happen externally...