They have large selection and because third party vendors can store their items in Amazon and fulfillment centers and get it shipped via prime. Unless Target also does that, Amazon will always have a much larger selection of items (like 1000 types of iPhone X cases).

Walmart offers third party selling but not as streamlined and its not shipped by Walmart. Just another ebay store.. If you have a problem with an order, you deal with Amazon and not the vendor.