I have observed recently that sites like Amazon have made some changes and the Add to Cart button now appears only when the user clicks on the thumbnail and goes to the detailed product page. Does anybody know why?
Most likely because product options/variations cannot be selected from the product thumbnail itself. Defaulting variations in color, size and quantity when a user clicks "adds to cart" from a product thumbnail page could really be a UX/Customer care nightmare if the user is not aware or notified product variations exist or that they can be modified. Leading to the wrong product being shipped to the customer. (Example: Customer thought they ordered a 16oz coffee cup but instead received a 10oz coffee cup)
In the long run, having the "add to cart" button on a product thumbnail ultimately can create more frustration than convenience for the user because the variations and product details are hidden. The user should have the necessary details visible to make an informed purchase. This helps both the user and Amazon with returns and incorrect/mistaken orders.
Purpose of Recommendation
Get the attention from user by using a thumbnail image and attractive price.
Other details like specifications, features, add to cart, same day availability, product variations, color selection cannot be displayed directly.
Not showing add to cart, enables customers to go through the product by navigating and add them to cart