Unless my own FAQ is mistaken, development on Divinity 2 started in 2004, which puts it in the same 4-5 year range of development time.
Any project comes with lots of documentation, discussions and making sure every stakeholder is kept happy. Even agile approaches need all of that, although they could give better time to market results. I never developed a game myself, but managed a lot of projects and don't really see how it can be done different.
As far as reinventing the wheel is concerned; in my personal observations there are those developers who are unaware of the existence of such a wheel, those that keep on searching until they find something that resembles it and there are those who suffer from the 'not invented by me' syndrome. The latter ones want to develop everything themselves as they feel what they make is indefinitely better than what is currently available. There is no known cure for these developers
Fortunately every now and then they do come up with something that really is better, that would not have been there if they re-used an existing wheel.
(Disclaimer: I don't have a clue about the developers at Larian Studios, they could all be divine coders as far as I know :-D)
