Sellers need to sell there to survive

Amazon is a service provider. Sellers sell there because Amazon provides product advertising (every product page is essentially an ad), order processing, payment processing, warehousing, order fulfillment (via the warehouse staff), shipping, dispute resolution, return processing (which is its own logistics nightmare), and even resale of returned/refurbished products in some cases, and all of it is coordinated through their data systems.

It is extremely convenient to sell a product on Amazon because they handle all of the customer-facing parts of selling, all you have to do is describe what you’re selling, and arrange for Amazon to get the product somehow. It’s the convenience that keeps sellers on their platform. It’s the convenience that makes it worth the cost of doing business with Amazon.

Now yes, each individual service could be replaced, but splitting them out is going to cause coordination problems. It’s going to slow down the order fulfillment, and it’s basically shunting the operation cost (both time and money) back onto the seller. That’s going to mean fewer sellers interested in using the alternative, because now they have to do for themselves what they could simply pay Amazon a percentage of their sale price to do. And because this alternative is slower and can’t provide the same kind of return guarantees that Amazon can, fewer customers are going to want to use it.