Here's why:
* The vowel sound in "prune" is a short "u" sound. It sounds like the "u" in "cup" or "duck."
* Long vowel sounds are typically associated with the vowel sound being pronounced as its letter name (e.g., "a" in "cake," "e" in "tree," "i" in "bike," "o" in "go," "u" in "cube").
So, while the "u" in "prune" is spelled with a single vowel, it's pronounced with a short vowel sound.