Why are there no product categories like fruit or dairy?

Because stores are more useful than abstract categories. pon groups by the shop you buy in and, within that store, sorts by your actual order — which mirrors your real path through the aisles. A fixed category order („fruit“, „dairy“), on the other hand, almost never fits the specific market you're standing in.

The benefit for you: instead of walking back and forth, you check off from top to bottom. And because an item can sit in a different spot at different stores, pon learns this separately for each store — completely automatically.