The researchers hypothesize that there is a physical Pavlovian response going on: When the product is actually available, that acts as a conditioned stimulus and elicits a response. The images and even text could become a conditioned stimulus and produce the same response, but they have not been set up in the brain to trigger the same response as the actual item.

https://www.blog.theteamw.com/2010/11/11/100-things-you-should-know-about-people-47-people-value-a-product-more-highly-if-it-is-physically-in-front-of-them/