Fashion may not be thought of as related to a million-dollar painting. But fashion is art because without the elements of art, fashion would not exsist.
For example, plaid would not be around without art because plaid is an example of a pattern, which is an element in art.
Color is also an element of art. Without this element in art, everything would be white, which would be boring.
Sometimes fashion involves buttons or other things you can feel. This is called texture. Without it, buttons would not exsist.
T-shirts with words on them involve an element in art called line. This is essential to all writing, which means that without this there would be no written language.