Here's why:
* Proper nouns are specific names for people, places, or things. They are capitalized. Examples:
* John, London, Amazon River
* Common nouns are general names for people, places, or things. They are not capitalized. Examples:
* boy, city, river
"Hide and seek" is a common noun that refers to a general type of game.
Note: If you were referring to a specific instance of playing hide and seek, like "The game of hide and seek we played at the park", then "hide and seek" would be a proper noun because it's referencing a specific event.