I can't recall what I was either watching or reading about exactly this, but the lady who wrote for bio shock and some guy that does assassins creed claim story is almost always the lowest priority when games are being designed, and some hardly get around to it at all.
I agree though, I looooove a fleshed out world. It's why I've become obsessed with the lores of warcraft and Tamriel among others, and I definitely enjoy a game more with a strong narrative.