I think it's partly because in the generally accepted, narrow view of disability, needing that spot means you simply must be a WC user, thus unlikely to be using a shopping cart on your own and would have someone pushing and returning the cart for you.

Also, if your stores are anything like mine, the carts are just sort of rolled in the general direction of the return areas. A degree of thought might have gone into trying to prevent those carts from being unintentionally or carelessly rolled into the spots or the WC spacing next to the actual parking area. Of course, since there is no return and those who do push their cart are often likely to leave the cart in the space anyway, that would kind of defeat the purpose of not putting a return area there.