Because Wikipedia's origin had a bad rap: initially anybody could post bogus information. It wasn't until later (after the site's credibility took a hit) that people were required to cite their sources (which people overlook). But even then, there's still a question of validity with the sources. With sources, you have to understand the author's rhetoric which most people fail to do thus interpreting a source incorrectly.
But that's generally speaking, as for Wikipedia directed towards anime/manga, what's there not to trust about it. No matter what, it will always boil down to what people think. For example, if in the manga it said, "Kakashi was hailed as the best shinobi in Konoha for 32 years - hokages included" wikipedia may interpret that as "Kakashi is better than everyone else" (maybe not that blunt) and they will cite the aforementioned manga page and people will still disagree and claim this information to be false.
