Why does an echo fill an empty room, but not one that is fully furnished?

In an empty room, the hard and solid walls, floors and ceilings act as good reflectors of sound, so the sound waves reflect repeatedly off these surfaces, creating echo.

In a fully furnished room, there are usually soft, hollow surfaces, like the fabric sofa; the thick curtains; the wardrobe, etc, that act as good absorbers of sound. This reduces the effect of echo.