Here's why:
* Short vowel sounds are typically short and quick, and they are often represented by a single vowel letter.
* Long vowel sounds are typically longer and are often represented by a vowel letter followed by a silent "e" at the end of the word, or by a vowel digraph (two vowels together).
In "cop," the "o" is short and quick, and it is not followed by a silent "e" or a digraph. The sound is similar to the "o" in "hot" or "dog".