Probably because it's all people really know if they're new to the post-apocalyptic genre.
However, nearly everything remotely post-apocalyptic can be connected to Mad Max and/or A Boy and His Dog.
Or "The Road", which does an even better job of describing the vast desolation, the mistrust, the drive for survival against overwhelming odds, the desperation of finding food and safety, and the overall fall of mankind. There is a lot of "The Road" in FO3, but that book isn't fun, and the man and the boy lead a life that no one would care to lead, (on it's face, anyway) and all of that would make for poor gameplay.
I'm rereading the book know, and after my last play of FO3, I was thinking that the big difference between the two is the type of heroism between the characters of the book and the PC in FO3. Going in guns blazing and killing everything in sight is the basest sort of heroism. Trudging along, mile after mile, trying to save your kid as you feel time running out on you is real heroism, IMO.