We often mark items as "Pre-Order" and "Back-Order" when we run out of stock and are planning to get the item back in stock soon. These items can still be put in the cart and ordered. Therefore customers often overlook the stock status on the product details page and think that if a product is in their cart and can be purchased, its immediately available to ship. If the stock status / availability showed up on the shopping cart page, it would draw more attention to whether the item is currently in-stock and ready to ship or not.