Here's why:
* Short vowel sounds are typically represented by a single vowel letter, and the vowel sound is short and quick.
* Long vowel sounds are typically represented by a vowel followed by a silent "e" at the end of the word, or by a vowel followed by a consonant.
Since "toe" has only one vowel letter ("o") and the sound is short and quick, it is considered a short vowel sound.