Here's why:
* Regular plural nouns form their plural by adding "-s" or "-es" to the singular form (e.g., cat/cats, box/boxes).
* Irregular plural nouns have unique, often unpredictable plural forms (e.g., child/children, man/men, city/cities).
In the case of "city," the plural form is "cities," which doesn't follow the standard "-s" or "-es" rule.