Here's why:
* Long vowel sounds are typically represented by a single vowel letter followed by a silent "e" at the end of a word (e.g., "cone", "note", "hope").
* Short vowel sounds are often represented by a single vowel letter followed by a consonant (e.g., "cat", "dog", "hop").
In "cost," the "o" is followed by the consonant "st", which makes it a short "o" sound, similar to the "o" in "hot" or "lot".