That and brand loyalty. If shopping was just api based, you could have your ai agent just buy X product from wherever when the price was right and never care about the brand or marketing or anything.

It would be hugely empowering to be able to make a non website based shopping list and just have “something” sort out all the logistics, biased towaeds reducing your costs and inconvience, but that is never going to be what even the ai companies are selling. They will funnel you to their “prefered partners” and find every possible way to extract money in the process.