Here's why:
* The schwa sound is a neutral vowel sound, often represented by the symbol ʌ. It sounds similar to the "uh" in "but" or the "a" in "sofa".
* In "fortune", the "u" doesn't have its usual "oo" sound. It's reduced to the schwa sound.
So, even though the word is spelled with a "u", it's the u that makes the schwa sound in "fortune".