Many reasons, but most important are that:
1. It's a known fact that Google hire search quality reviewers who manually assign results to some of the most important queries, either to fix some of the bugs/problems with the algo, fight spam, or simply to present more relevant results that searchers are looking for.
2. There is a lot of money in the serps and as any publicly traded company Google need to make more revenues to keep their shareholders happy, so the pressure is high.
3. Many seo experts I've talked to believe so. And I don't mean those who just call themselves seo experts, but people who are successfully competing for some of the toughest keywords in Google and have an inside scoop on the industry.
4. There is virtually no risk in it for Google, no one can ever prove anything without having access to the algorithm.
5. Google is evil

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