That could be for a variety of reasons. Is that site the only one that is offering that particular product? Is it a highly trafficked site with a lot of backlinks, reviews, and online activity? Are the pages simply coded properly using canonical tags which help them escape "wrath"? These are all valid questions when you're doing competitive analysis and all things that Google considers along with dozens of other considerations.
Your best practice is to create new descriptions, take new photos or alter the existing ones (add text, crop, change contrast, etc.). This way your listing is seen as fresh and original content and will eventually take precedence over their carbon copy approach. If you have a better page with better content that's more informative to the customer, Google will choose your listing over 20 other sites that all have the same photos and descriptions.
Originality always wins....in most cases. Keep in mind that there are many other considerations in the Google algorithms, so don't expect to beat out Amazon no matter how hard you try. 