When Google launched its AdSense program, it did two things: It created an infrastructure whereby the company could bring in billions of dollars in ad revenue, and it created an opportunity for thousands of small businesses to go into web publishing.
In that latter opportunity though, began a major problem. Rather than building a web publishing business around creating informative, quality content; thousands of get-rich-quick innovators decided to game the system. The predominant business model of web content, especially in the early days before Google started refining its algorithm, was to produce quick-and-dirty, marginally useful and usually poorly-written content pages stuffed with keywords and Google ads. The business model held that even if content was poor, the presence of ads would encourage people to click through to find something more relevant.
The large and growing community of freelance writers and production shops was of course appalled, since the aforementioned get-rich-quickers built their businesses around acquiring content at a penny a word or less, and the writing end of the business quickly went either offshore or to kitchen table amateurs looking for beer money. In most cases the beer was obviously consumed prior to writing.
The poor quality of web content quickly proliferated—because it was making money for the publishers. Pay for "writers" continued to decrease, because after all, the writing wasn't meant to be read by humans—it was meant to be read by a computer algorithm. And because the writers were writing for computers and not people, it could be done to a formula. Orders for articles called for specific keywords to be included to reach a certain penetration level, so that the algorithm would think that it was more relevant to the topic at hand. Never mind the fact that every sentence had to include the phrase "toenail fungus remedy" and it was painful to read, the fact was, if the article conformed to spec, it would land prominently on the Google search engine results page.
This business model did lead to thousands of small web publishing operations making money, and a handful of very large ones as well, all built around commoditized writing spun out on a regular basis for an algorithmic, non-human audience. But let's look at the trend and into the future for the next year. This business model simply isn't sustainable over the long run. A business model built on poor quality, when better quality is otherwise readily available, will not work for very long.
Google's last major update in April 2012 did close up some of the "Black Hat" SEO techniques that have for so long degraded the quality of the Web, and Google's Matt Cutts gives some great examples of some of the more laughable content pages they're trying, justifiably, to push down.