[bjourne]

It's because Wikipedia is edited by anonymous people. sources is not to say "this is true because this is written in this source" but "this is true because someone with authority said so". Citing sources is is a form of argument from authority. It is a a fallacy if the source has no authority but a completely valid argument if the source does. Wikipedia has no authority because of its anonymous nature. There's no one to hold responsible and no one who'll lose credibility by publishing bullshit on Wikipedia.

Also, most Wikipedia articles worth citing contain really bad factual errors. Since most readers aren't topic experts they don't notice such errors. But they are there and they are really bad. I only have some math education but I've found lots of mistakes in proofs on Wikipedia. Imagine how bad it is for higher-level math that is beyond the reach of most visitors to the site.