They licenced the rights to Fallout 3 in 2004. I'm fairly positive the preproduction phase started shortly after that. And seeing that the basics of their mechanics came from Oblivion, they didn't really do everything from scratch.
But an complex, story driven, open-worlded RPG? [censored].
Sure, they had Gamebyro engine. That still doesn't mean 80% of their work was only goofing around with the scripts. They had to rewrite the armor system, add new stuff, hire MUCH more voice actors, including some celebrities and heck, do the whole audio production, either re-use or think of some new concepts, have ACTUAL writers do their job, think of a new plot etc.. and some effort might have gone into marketing stuff too.
Now to code and programm that [censored], get past the "Playable", "Alpha" and "Beta" stage , Quality Assurance and prepare for the Gold build.
2 bloody years is definetly not enough, for an RPG. See Dragon Age 2.
Exactly.