Here's why:
* The "ou" in "sprout" makes a short "ow" sound. This is a distinct vowel sound, not a long vowel like the "o" in "go" or the "u" in "flute."
* The "o" in "sprout" is actually silent. It doesn't contribute to the sound of the word.
Therefore, "sprout" has a short vowel sound, not a long vowel sound.