Here's why:
* Many "-y" nouns are non-human: Think of words like "city," "story," "journey," "history," "melody," "energy," etc. These clearly don't refer to people.
* Some "-y" nouns can be either person or thing: "Secretary" is a person, but "bakery" is a place. "Enemy" is a person, but "enemy" can also refer to a concept or a thing (like a disease).
* "-y" can be a diminutive: "Puppy," "birdy," "mommy" - these often refer to a young or small version of a person.
So, the ending "-y" isn't a guaranteed indicator of a person noun.
It's best to look at the context to understand what a specific noun refers to.