Search Engines such as Google have their own algorithms. Little is known about how the algorithms are calculated and Google has never publicly stated which algorithms it uses. Therefore, people are not able to beat the system by filling their site or pages irresponsibly to raise their ranking on the SERPs. The algorithms are regularly reviewed and refined to prevent any new methods that are used to cheat the system.
To quote an example, at one point it was discovered that one of the factors of Google’s algorithms was to repeat keywords in a blog. This caused many bloggers to spam the keywords in white, in-between paragraph spacings. The keywords were detected by bots but not seen by the reader. This resulted in many blogs ranking high not because the contents were useful but because the keyword was repeatedly mentioned.