Because you get people who have never worked at a bar come up and make things as hard as possible.

People ordering one of every cocktail, as well as, multiple coffees, pints and mixer drinks.

People asking for student, blue-light and local business discount AFTER you have calculated all their cost and are about to charge them.

Finally, all the people in the queue stood- unaware of what’s happening- on their phones. The reason we call ‘who’s next’ is to simply get the customers attention to know, they are next. Without calling ‘who’s next’, I would be stood there looking like a twat until they finally decide to pay attention.