Amazon had an aha! moment somewhere during the evolution to what it is today, which this video explains quite well. It runs contrary to conventional thinking about warehouse organization, because Amazon uses computers and barcodes extensively.
When Amazon receives a product from a supplier, they break the case apart and store each item randomly all throughout the warehouse, which means when a person orders more than one item, the walking time to gather the items goes down. Even if it is a newer warehouse where robots are doing the moving, it still saves time. Not having fixed locations for certain merchandise means the space adapts to what people are currently ordering.