Because -
(a) The Civilopedia isn't just there to give workaday historical descriptions but to add interest, at least in my opinion. It ought to form an organic whole, with the different entries all being written from the same point of view and for the same purpose. Just cutting and pasting soulless paragraphs from the Internet seems to defeat the whole purpose of it. You might as well give page references to the Encyclopaedia Britannica.
(b) Wikipedia is utterly unreliable.
(c) The Civilopedia is the one part of a mod that anyone can do. You don't have to have any graphical skill, programming knowledge, or ability to comprehend the arcane workings of the Editor. So when people don't put any effort into the easiest part of the mod, it makes me rather suspicious of what the rest of it must be like!
Just my view, anyway. Personally I'm always disappointed when otherwise great mods have patchy or uninteresting pedias, but perhaps other people aren't so bothered by this.