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There are companies out there that have PEOPLE solving all those CAPTCHAs for basically nothing. For example, KolotiBablo is one company that exists for just for spammers, and offers CAPTCHA solving at low, low prices. For ~$0.75 per thousand CAPTCHAs solved, they will have some poor sod in Pakistan, China, Vietnam, etc. sit around all day typing in solutions for a couple of dollars per day. Having actual people doing the work makes the solution rate higher than programs that read CAPTCHAs, and the cost is low enough to keep CAPTCHAs from really discouraging spammers.
I ran across an interesting article on this subject recently. I had never thought about digital sweatshops before, but that's a pretty apt description of how all those CAPTCHAs get solved.