Because Amazon wants to have the best price overall for each product, and the way to do it is to have many sellers competing on price alone for each product. T

Because Amazon wants to have the best price overall for each product, and the way to do it is to have many sellers competing on price alone for each product. Then commingling is just a logistics trick, it makes inventory more liquid and so lowers costs and shortens delivery time. Amazon likes to think of itself as the arbiter of a fair marketplace between sellers. It's not that they don't care, it's that they are proud of doing things the way they are.