Here's why:
* Short vowel sounds are typically quick and sharp. Think of the "e" sound in words like "bed," "pen," or "set."
* Long vowel sounds are usually drawn out. They are often spelled with a vowel followed by a silent "e" (like "name," "bike," or "kite").
The vowel sound in "hey" is more like the "e" in "bed" than the "e" in "bike."