Probably a few things:

1. When you visit amazon.com looking for a product, even if you are looking for a very specific product, they'll put other products in front of you. You might end up buying one of these other products, and that might make someone more money than if you bought the product you initially planned to buy.

2. You might buy more than you initially intended if you visit the site yourself, via the "Customers Also Bought" links, and other advertising.

3. Agents aren't perfect, and it's possible that they were making ordering mistakes at (much?) higher rates than humans were doing, which was costing Amazon more processing returns or other customer service inquiries.

4. Amazon wants to control the experience of buying a product through them, from start to finish. If they can't do that, they just become a commodity product aggregation site. An agent making purchases for someone can find the same product from another retailer, perhaps with better pricing, faster shipping, a better return policy, etc.

Of course, if other sites allow agents to make purchases, and that eventually becomes the way a significant number of customers want to shop, Amazon will have to get on board.