Here's why:
* They refer to unique entities: Proper nouns name specific people, places, or things (e.g., John, London, Earth). There's only one John, one London, and one Earth.
* They don't have plural forms: You wouldn't say "two Johns" or "three Londons."
* They are singular in nature: Even when referring to multiple people, you still use the singular form (e.g., "The Johns are coming over.")
However, there are some exceptions:
* Some proper nouns have plural forms: For example, "the Americas," "the Alps," or "the Smiths."
* When referring to specific instances of something: For instance, "There are three Johns in my class."
In general, though, treat proper nouns as uncountable.