Here's why:
* The vowel sound in "stay" is a short "a" sound. It sounds like the "a" in "cat" or "hat."
* Long vowel sounds are usually associated with a vowel that is spelled with a single letter followed by a silent "e" at the end of a word (like "cake," "hope," or "time") or a double vowel (like "feet," "rain," or "moon").
While "stay" has a silent "e" at the end, the "a" sound is still short.