This makes perfect sense. What Pinterest offers are not links to products, but a stream of aesthetically-pleasing visual stimuli that are constantly new and interesting, but fleeting.

Their other major product is self-satisfaction. The act of re-pinning is a way to prove your taste to others, and when your pins get re-pinned or liked, the reward in terms of self-aggrandizement is fairly substantial.

Could they do better to convert people to purchasers? Probably. If there was a buy link on many of those images, I'm sure people would click it and consider. As it is, they're not trying for that, because they're not in the business of selling products; they're in the business of providing emotions.