Here's why:
* The "i" in "zipper" makes a short "i" sound, like in "zip" or "bit."
* Long vowel sounds are typically spelled with a vowel followed by a silent "e" at the end of the word (like "hope" or "bike") or with a double vowel (like "moon" or "rain").
Let me know if you'd like to explore other vowel sounds in words!