Why is there no change in water level when a block of ice melts in a beaker of water?

Actually, whether that is true depends on the temp of the ice. At exactly 0 degrees C / 32 F if you did this with a very very large ice cube the water level would actual drop slightly. Ice floats because it is (slightly) less dense than water. So a block of ice will melt into a slightly lower volume of water. So the water volume will drop (very very slightly when the ice melts! But unless you have a large amount of ice it will be too small for you to measure.