text: Because it is plural.  Try any other country and watch what happens when you say The __________countries. ie. Europe and The European countries. Africa and The African countries.\n\nTHE is also used when something is one of a kind and you know what is being talked about ie. The sun The moon (in our own gallaxy system - A sun a moon where there are many rotating around a planet.\nIn daily life if I asked for THE RED BOOk on the shelf. there would be only one red book on the shelf. If there were many I'd have to add a descriptive word such as THE THICK RED BOOK or if I didn't care which book but only wanted a red cover book then I'd ask for A RED BOOK and again if it were many I'd ask either for SOME OR THE OR MANY.\n\nIn th case of the countries you mentioned since they are plural the word THE would be used. Some or many would not stop word THE from being used in referring to these countries.  Just noticed that THESE (plural form) could also be used to plurel countries but adding a noun ie These European countries These United States or these USA states, these Netherland countries. Note the s tranfers to the noun from the two countries you asked about.