I think Amazon wants to protect their position as the go-to place for shopping. The place where people start when they are looking to buy something. Amazon are able to capture value from that through seller fees and advertisement, and they had revenues of $20 billion from advertisement in Q2 2026. There was a recent discussion on this "Amazon tax"¹

If people initiate their shopping through their agent instead, the agent can search any merchant and find the preferred combination of product, price, and delivery. Amazon lose their position as starting point for shopping, and that puts their seller commission and advertisement revenue at risk.

Instead the agent providers will own, and could monetize, the demand.

[1] https://news.ycombinator.com/item?id=49345263

So Meta wants to put a store within the Amazon store, in short, benefit from the infrastructure and pay nothing, or very little.