When talking on a cell phone you enter a kind of mental bubble. Your brain's speech centers are so large that, when operating, they drown out or blunt other mental operations. You don't really notice the people around you. You're in a phantom phone booth. We should also note that cell phones haven't yet developed an etiquette.
People still have to be told to turn off their ringers at, for example, funerals.
Technology comes first, then manners. It's like when they invented the first crossbow it wasn't a good time to be walking the streets at random. The fact that it emulates the football goalposts, and is used to signal an accurate kick, is worth pondering. But let's go with the anthropological answer this time. Arms upraised is known as a Triumph Display.