Because they make a shitload of money by arranging Amazon search results in a certain way, selling favorable placement on those results, and inserting upsells ("Try Amazon Prime!") into the checkout process, all of which are at risk if Chat-GPT becomes a frontend to buying on Amazon. (Note that they are already vigorously going after Perplexity for trying to be an Amazon frontend)

In general, Big Tech will never allow itself to be just the backend to a service where another company controls the frontend and the relationship to the customer. That's how you get commoditized and ultimately replaced.

Examples: you cannot get a streaming box with universal search ("which streaming service has show X? Just hit play and go"): the streaming services staunchly refuse to provide the APIs to do so. Nor is there interoperability across messaging apps to let users supply their own frontend clients. AI and MCP will go much the same way, it will be locked down as soon as it presents a business model threat.