Why does wattage alone not predict real speaker performance?

Wattage describes electrical power, not the complete listening result. A speaker's output also depends on driver sensitivity, cabinet design, frequency response and distortion. In a powered speaker, the amplifier, processing and protection settings also affect usable output. Power figures may describe different conditions, such as continuous operation or short peaks, so they are not automatically comparable. Ask how the rating was measured and review maximum sound pressure level, tonal balance and clarity at the intended listening distance. Compare samples at matched levels rather than judging performance from the largest wattage number.