Properly done each fact (or set of facts) will point somehow to the source, including the page in the source.
How? And how will the article still be readable? And what is a fact? And what use is the page number when there are loads of editions of a source?
No encyclopaedia is written like this.
I agree that it would be difficult to achieve this, and far too labor- intensive. (This was, from what I've heard, one of the major flaws of Nupedia.) From today's featured article:
Until the mid-19th century, the area around Kalimpong was ruled intermittently by the Sikkimese and Bhutanese kingdoms. Present-day Kalimpong is believed to have once been the forward position of the Bhutanese in the 18th century, overlooking the Teesta Valley. The area was sparsely populated by the indigenous Lepcha community and migrant Bhutia and Limbu tribes. After the Anglo-Bhutan War in 1864, the Treaty of Sinchula (1865) was signed in which Bhutanese held territory east of the Teesta River was ceded to the British East India Company. At that time, Kalimpong was a hamlet, with only four families known to reside there. The first recorded mention of the town was a fleeting reference made that year by Ashley Eden, a government official with the Bengal Civil Service.
I count nine statements of fact there, and no specific citations or sources. Do we really want to have, at minimum, one footnote per sentence? Unless MediaWiki gets fixed fast and we can have, say, implied metadata for any piece of text in an article, it would be unreadable.
That said, it would be nice to eventually have such a system.