Here's why:
* Short vowel sounds are typically found in closed syllables (syllables ending in a consonant). In "cake," the "o" is followed by the consonant "k," making it a closed syllable.
* Long vowel sounds are often found in open syllables (syllables ending in a vowel). If the word were "cakey" (with an extra "y"), then the "o" would have a long vowel sound.
Therefore, the "o" in "cake" is a short vowel sound.