Here's why:
* The vowel sound in "bones" is short: It's the short "o" sound, as in "hot" or "dog."
* Long vowels typically have a sound that's the same as the letter's name (like the "a" in "cake" or the "e" in "tree").
While the "o" in "bones" is doubled, this doesn't make it a long vowel. The doubling affects the sound of the preceding consonant ("n") rather than the vowel.