In many cases Wikipedia's source links are dead. And nobody bothers to update them. Or they cite some random book that I have no way to get and read myself.

Who checks if that book actually exists and is not made up? Especially if it's not a highly politically charged topic.

And then there are circular citations. Someone posts unverified/false info on wikipedia. Then it gets cited by some blog or other media that wikipedia considers trustworthy. And that then gets added to wikipedia as a reliable source.