Here's why:
* Pronouns are a part of speech. They stand in for nouns.
* Antonyms are words with opposite meanings.
Therefore, an antonym would need to be another part of speech that has a directly opposite function. There isn't a grammatical category that functions in a directly opposite way to pronouns.
However, you could think about the concept of pronouns and find words that relate to the opposite of what pronouns do:
* Noun: A noun is what a pronoun replaces. This is the closest you'll get to an opposite in terms of function.
* Proper noun: If you're thinking about the specificness of a pronoun (like "he" vs. "John"), a proper noun could be considered the opposite.
* Full name: Similar to the above, a full name could be considered opposite to a pronoun in the sense of specificity.
Ultimately, the best way to think about "pronoun" is not in terms of an antonym but in terms of its function within a sentence.