For the very same reason, it has earned a lot of criticism - namely by people who feel its anonymous nature makes it too unreliable and inaccurate. Many have accused the reference site of this, and as a result, Wikipedia has often found itself fighting an uphill battle to prove its credibility.
In an effort to help passersby distinguish the verified material from the untrustworthy, Wikipedia is preparing to introduce a new tool. A new proposal, made possible through an extension to the MediaWiki software that Wikipedia is based upon, will see unreliable text highlighted in color.
The new system will give contributors a reputation score between zero and nine. When someone makes an edit, the resulting text is then highlighted with a shade of orange - the brighter it is, the less trustworthy. An editor's reputation is influenced by the performance of their contributions. As articles become more stable and presumably verified further, text would revert to the norm.
The new system isn't foolproof - it still relies upon anonymous editing, and assumes that editors who have been around longer are more likely to give you accurate information. That may be a safe assumption - the overwhelming majority of vandalism to Wikipedia comes from shadowy sources.