Do you ever think about how strange some social customs are? For instance, the practice of tipping the server at a restaurant. There’s no reason why restaurants couldn’t simply build the correct compensation for the server in to the price of the meal. In fact, sometimes they do. But we nevertheless continue the practice most of the time, even though it is not only inefficient, but also places the server at the mercy of the customer’s whims, which may be quite ungenerous or unfair.
There are other criticisms of tipping on economic grounds, some of which you can read about here. (The Wikipedia article also cites this rather good article on the matter.) It’s not a big deal, of course, but it strikes me as a very unsatisfactory way of compensating people. But nobody really thinks about it much, commonplace as it is.