[ProDigit]

If such a site exists,we call it the internet!
And it does exist.

But if you mean by 'all knowledge',you mean 'ads', then no!

If you mean sites like wikipedia, but more expanded;yes; by all means,let them exist.
But only if they reduce ads to a minimum, or non visible, and can provide content to users according to the USER's preference, not the website's preference.

That also means that if the website becomes a monopoly, it needs to do a good job on keeping everything clean, pleasant for the user, and directed towards the interest of the user, not their own goals or plans; all the while still keeping some form of privacy to be allowed on such a website.

I'm literally thinking about a site which could be the combination of wikipedia/dictionary, facebook, google, and amazon/review site!
A social network combined with an extraordinary search engine, huge database,and items for sale connected with staff/user reviews.
Though I would not know how to tie all of this together in one site.