Why use a plural article in, for example:
- les États-Unis
- les Émirats arabes unis
- les Pays-bas
The names of the countries mentioned in the question are based on a composition of units, therefore the article is plural. However, most of the countries are "singular". The other exceptions include countries composed of many islands: Les Seychelles, les Philippines...
Les États-Unis d'Amérique (federal republic composed of 50 states)
Les Émirats arabes unis (Federal monarchy composed of 7 emirates)
Les Pays-Bas (historically, 7 independent provinces united to create the Provinces-Unies, which evolved through the time to become today's Royaume des Pays-Bas)