I guess for most people it's not the money, but the feeling of accomplishment and the sense of community - teaming up to achieve a bigger goal. I doubt that sweatshops would do this - they are busy breaking captchas and mining gold in WoW.
Except, how do you know what the bigger goals are? I signed up for MT a while ago thinking I should give it a shot to see what it was like. I had no idea what the tasks added up to on a grander scale and I found that it really wasn't worth my time.
Edit: Are you sure that folks don't use MT to break captchas?
Efficient Frontier, a search engine marketing firm, has used Mturk.com to accomplish tens of thousands of tasks since early 2006. Efficient Frontier helps companies figure out which keywords will bring Web surfers to their sites. With Mturk.com, Efficient Frontier can afford to pay three different people to look at each potential keyword, and vote whether those words are relevant to a given site. It costs the company just 4.5 cents to test each keyword, paying 1.5 cents to Amazon, and 1 cent each to three turkers.
