Yeah I know what you mean! My guess is it's budget intensive and boring. Imagine a nuclear fallout and somehow you survive... what are you going to do for the next 3 years? probably die in some slow and painful or unexpected way. Wouldn't make for much of a show. 

I heard somewhere that the best tv is about people. I don't know if I agree because the people stories can get very annoying - like all the talk in the walking dead, they just don't shut up! I'd love to see the science of it as well, but I do wonder if those sayings are true - if the action would be too boring to most people.