So why have we stopped building pyramids for thousands of years? We have the technology and knowledge to do it, so where are all the new pyramids? And no, that Las Vegas monstrosity doesn't count.

You might be surprised to learn that not all challenges are purely technological. The biggest challenges with such megaprojects are political, economical, and social. A Wikipedia clone is completely worthless if nobody is willing to contribute - it would just be an empty database!