As others probably mentioned, the army military can't be everywhere and while irl plans to fight a zombie outbreak do exist (probably not mentioning zombies but describing a virus or such with the same effect) obvious the military is not able to save everyone because in all countries the military is dwarfed by the rest of the population and the country's size means travel takes a while, even more so when lots of infrastructure and vehicles, etc. are not usable or require clearing of some manner or other due to the zombie outbreak.
Like in most zombie stories most likely there will be several outposts run / protected by the military in an unprotected wilderness and anyone not able to right away reach such an outpost must try to stay safe where they are, e.g. if travel is too dangerous, or try to reach an outpost with the military maybe assisting them if they are lucky but most likely not having the capacity to do so because they are fighting of zombies at the outposts, transporting large amounts of refugees elsewhere, etc.
Not to mention any large organiziation liek the military is an easy target for the spreading of the zombiefication in the first place so possibly only a tiny amount of military personnel are still humans .
PS: for the love of god not zombies again, they are a beaten and decayed horse of which barely a skeleton remains and really boring by now.
At least make them interesting zombies that do more than just run fast or are kind of smart.
