Here's why:
* The vowel "u" in "humor" is short. It makes the sound you hear in words like "cup" or "sun."
* Long vowel sounds are typically indicated by a vowel followed by a silent "e" at the end of the word (like "hope" or "bike") or by a double vowel (like "moon" or "rain").
So, while "humor" has a vowel sound, it's a short vowel sound.