It is well known that people seek out information that reinforces their own beliefs. And while it may not be ideal for an enlightened population, why should Google/Bing fight this natural inclination? Their job is to ultimately give their users what they are searching for, not challenge their users' opinions. Especially since there are tons of benefits to personalized search, and it may be hard to not throw the baby out with the bathwater.
This mentality ("give people what they want") results in the empty, sensationalist journalism that most people dislike. When I read the news, I don't want to be entertained, I want to be informed. The same goes for Internet searches.
