That is your interpretation of shopping carts in web stores. Other people add things to their cart as they need to replace eg: items in the stationery cupboard, then checkout once a week. Some people use the cart as a form of Wishlist, or a picking list while evaluating similar products.

At the very least if a site doesn’t offer Wishlist, shipping list or other bookmarking facilities I would expect the shipping cart to give me a cookie that lasts at least three days to cover the weekend or the option to create an account to save that shopping list/cart to come back to later.