Here's why:
* Sound: The "y" in "silly" makes a "y" sound, not a vowel sound like "a", "e", "i", "o", or "u".
* Position: The "y" in "silly" comes before a vowel, and in this position, it typically acts as a consonant.
While "y" can sometimes act as a vowel (like in "my" or "cry"), in the word "silly", it's functioning as a consonant.