The reason: there's a lot of money in fake reviews, according to an excellent exposé by the New York Times's David Streitfield.
Take the case of Todd Rutherford, for example.
Eventually, Rutherford realized he could cut out the middleman. So he started charging money to write reviews. He would charge $99 for one review, $499 for 20 and $999 for 50. He eventually published 4,531 reviews and at one point pulled in 28,000 per month.