Here's why:
* Language Evolves: Words and greetings change over time. There was no single "first" word or greeting that everyone used.
* No Written Records: We don't have written records from the very early days of human history to tell us what people said.
* Cultural Differences: Greetings vary widely across cultures. What someone says in one place might be very different from what someone says in another.
So, while it's fun to think about, we can't know for sure who the first person to say "hi" was!