Because Pinterest is not a platform for publishing art. It is comprised primarily of products and feeds directly into the consumeristic culture that pervades society. Those that are not products are nonetheless incredibly succinct and are displayed in such large volume and quick succession as to trivialize any artistic value derived from them. A picture of a latte or a beach in the Bahamas might be "art," but a thousand pictures displayed in a grid is superficial fetishization. It might be entertaining, and I don't fault anyone for enjoying Pinterest, but it certainly isn't contributing to the culture in a meaningful way.