It's not just Latin American countries but Canada as well and again, there's a good reason. For much of it's existence as a nation, the United States Foreign Policy has been based on some form of Monroe doctrine, which holds that the United States will deal with problems in the Americas and not make trouble in the Europe, Asia, and Africa unless they start problem first. It's easy to under-fund the military when the most powerful military in world says "Not in my back yard" and you're in the Backyard. It's also very difficult for anyone to invade a country in the Americas as it's separated by an ocean and very difficult to commit landing forces if you can't stage in an American overseas dependancy, of which, only a handful of countries still have and most aren't set up to handle that without the US noticing (the largest Border France has with any nation is Brazil). It should be notable that the US only fought three major wars against foreign powers in the Americas (The War of 1812, against Britain via Canadian Colonies), The Mexican-American War, which lost huge swaths of Mexican territory, and the Spanish-American War (which resulted in Spain losing most of its American Colonies).
That isn't as other people are pointing out reason to doubt other nations in the Americas. Brazil, for example, was the first nation in the Americas to field a post Dreadnaught-styled Battleship (which before nukes were a thing, was the weapon that sparked an arms race) and was Third in the world to have one (following Britain and Germany). At the time, there was a lot of speculation as to just how Brazil did that as it wasn't a nation known for it's navy before it launched it's battle ship and many thought that it only got one from stealing plans from either Britain or Germany (it in fact did not). Today, Brazil is anticipated to soon launch a nuclear sub, though it won't be part of any nuclear launch capability (no nuclear missiles), just help it be stealthy. Additionally Argentina was on the losing end of the only naval battle to take place post WWII and again, part of the problem was a nation that was only recently reduced to second best navy in the world (it was able to put up the fight it did in large part because Britain had so slashed it's Navy).
It's also important to understand that many South American Nations also have some uneasy distrust of the military as Military Juntas were quite popular Post World War II and didn't tend towards Communism, which also meant they weren't undermined by the U.S. which means they will let you play with their toys to some degree. Brazil was by far the largest of these nations and because of it's historically terrible navy, it didn't get to see much action in either world war despite being an Allied Nation in both of them. Modern Brazillians and military observers are quick to note that Brazil's greatest Military success was invading and holding itself (In World War I, the Brazillian fighting forces arrived in France mere days before the 11/11/18 Armistice was signed, ending the war. While they saw more action in WWII, by this time the citizens of Brazil so often joked that "You would sooner seen a snake smoking a pipe (which is their version of the "when pigs fly"), then the Brazillian Army at the front" that to this day, the BEF insignia is a snake with a pipe! Even when it was in control of the country!) Not only that, but since the military did have a lot of control in the back half of the 20th century in Dictatorship forms... most of them have ousted these leaderships in favor of a democratically elected government and fairly recently too. Most of the voting public in those nations are not likely to support a strong military any time soon.