The reason to implement structured data is to get a "rich snippet" in the Google search results. Google's search gallery shows what the snippet would look like for a product offer:
Information about a product, including price, availability, and review ratings
Having that gray line with the additional info in the search results can entice users to click to your site more often.
Structured data doesn't help with SEO in any other way. Google doesn't use it for ranking purposes. It just adds to the display in the search results when Google supports it and chooses to use it for your site.
In exchange for this carrot of a slightly enhanced search result, you you giving Google (and everybody else) information about your entire product catalog in an easily machine readable format. My guess is that Amazon decided that they would rather keep that information harder to scrape off their site and forego the rich snippet. Amazon has the advantage of brand recognition and great rankings. They may have found little or no increase in CTR from implementing structured data for rich snippets.
Regardless of what Amazon does, you need to make the decision in terms of what is best for your own website. Can you get rich snippets? Do they increase the CTR meaningfully? Is problematic to make prices and other product data easily machine readable off your site?