Because of 8th grade physics.
But, the global warming alarmist argument isn't so naive. They claim that the glaciers on top of land are also melting, which would be analogous to you adding another ice cube to your already full glass.
Their estimates of glacier volume are way off, of course. They don't account for new snowfall due to changes in weather patterns. They also wildly overestimate how much of this landlocked ice would actually melt; imagine thinking that any significant part of Greenland is going to melt because it got 1C warmer. Or 10C warmer. On average.