Why hasn’t someone done this before?
Because it takes time. It wasn’t until the 1970s that Babbage’s contributions were fully understood (see Bruce Collier’s thesis), and until the 1980s that his plans had been deciphered by the likes of Allan Bromley. Only then could the Difference Engine No. 2 be constructed. It was finished in 1991. In many ways, The Difference Engine No. 2 was an ‘easy’ project because Babbage had left complete plans for it. The Analytical Engine is a different matter.
Babbage left multiple plans for the Analytical Engine and was constantly refining its design up until his death. To build the Analytical Engine first requires a research project to figure out which plan to build from.