why do bartenders have to wear black?!
Answers:
They don't
One pub I worked in made me wear a horrible brown striped shift, every other place I worked in let me wear my own clothes.
My local has staff in white shirts, except the management who can wear any colour.
Another pub I go in has green, branded, polo shirts
Generally speaking they certainly do not HAVE to, but the bar they work at may make them. It is because the bartenders are not supposed to be the center of attention, instead they try to stay in the background. You will see them wearing black at places which want to focus on your experience at the bar and not just drinking there.