Here's why:
* Common nouns refer to general things or ideas. They are not specific to a particular person, place, or thing.
* Proper nouns are specific names of people, places, or things. They are always capitalized.
Examples:
* Common noun: world, country, city, river
* Proper noun: Earth, France, Paris, Nile River
Since "world" is a general term referring to the planet we live on, it is a common noun.