Here's why:
* "Short" is a noun that refers to a piece of clothing, and in French, most nouns referring to clothing are masculine.
* The plural form of "short" is "shorts", and the plural article for masculine nouns is "des".
So, when you are talking about shorts in French, you would say:
* J'ai acheté des shorts. (I bought some shorts.)
* Il porte des shorts. (He is wearing shorts.)
* Elle porte des shorts. (She is wearing shorts.)