Well, actually, if we waste time thinking of it now, we can save lots of time (and water, beer, etc.) later on. That's the point. Worked in pub for nearly a year, trust me, whether it's water or beer, if you want to pour it fast you have to have the glass tilted sharply against the tap and bring it down and to the vertical as it fills to prevent spill.
From the side, the centre of gravity appears as a cycloid. It also minimises agitation inside the glass so that if the drink is fizzy you don't end up with 20% liquid and 80% foam.