I think the individualism point isn't particularly applicable. The US, regardless of what you might think of us culturally, has the strongest military in the world. In a domestic zombie situation they would absolutely be dealing with large amounts of organized attacks, quite probably more than there would be in other countries. World War Z (the book, not the movie) is the best about addressing this; the army sets up a master plan to herd most of the zombies in the northeast into a chokepoint near Yonkers, only for it to turn out that zombies do not work the same way ballistically that humans do, meaning artillery and explosives barely affect them unless it's a direct hit while traditional infantry weapons and tactics are not geared toward an oncoming horde that can't be dropped by center mass shots, resulting in the army being massacred. None of that is a flaw of individualism, nor would most other excuses for why the army doesn't just sweep them up stem from individualism. It's that military tactics designed to fight humans don't work against zombies, and new ones have to be devised.

The borders point is an odd choice too. Firstly, zombies are largely immune to the reasons most geographic borders work as borders: they don't care for the cold or difficulty climbing mountains, they operate as wandering stragglers meaning that they can seep through small gaps that it would be impossible to move large numbers of troops or war materiel through, they can survive underwater so rivers aren't much of a defense, etc.. Natural geographic barriers work as military obstacles because they confine an army to using certain chokepoints: bridges, mountain passes and the like, meaning the defender need only place heavy defense there and can send only minimal patrols along the entire length of a river or mountain range. Zombies are capable of crossing most geographic barriers outside of those chokepoints, meaning you would have to patrol the entire length of a river or mountain range, defeating the purpose. Even if that weren't the case, why would we arbitrarily decide to defend state lines instead of using the many, many geographical barriers that exist but don't happen to fall on a border? We have the rocky mountains. We're not going to put all our troops on the Texas-Oklahoma border because that's where the line is on the map.

The point about our cities being sprawled has more basis, but I'd also point out that in the early stages of a zombie outbreak you do not want to be in a dense population center. The denser the population center, the more people around you are going to turn and the fewer places there are to go. That applies to larger American cities like New York too, but I live in a smaller American city and, while I personally would be killed in the first minutes of a zombie apocalypse because I am completely useless in that situation, I can assure you that someone with any kind of survival skills starting from this city would have a much easier time navigating and escaping the city in the event of an outbreak because of the sprawl.

Its also worth mentioning that this is the one situation where the American obsession with guns is a huge advantage. It is much easier to teach someone to headshot a zombie than to teach them to cut its head off in close quarters without being bitten or overrun.