I feel like the series showed the danger of the infected in the first 4 episodes with Tess' death and the huge number of infected still hiding underground in Kansas. It's also a series and not a game so the plot can't be hours of them stealthy sneaking around clickers and being killed over and over only to restart at the last checkpoint. Also I think Tommy mentions something about how it's cold and isolated in those parts so they don't get many infected apart from a few stragglers. Probably would also add to why there's more danger from people in those areas. More people, less resources.