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A transformer is an inductance which shifts current phase by 90 degrees (shifts the periodic waveform in time by 1/4 of the period) in one direction , and a capacitor shifts the current 90 degrees in the other direction producing 180 degrees of phase shift.
Can you see what happens to the actual power when the voltage and current are 180 degrees out of phase? They add up to zero. Power is proportional to voltage multiplied by current. Zero current is zero power. The currents flowing in the inductor cancel out the currents flowing in the capacitor.
(Feel free to poke holes in this, I didn't think through it very carefully before typing).
It's because when you apply an AC voltage to a capacitor (an ideal one), it absorbs no power. No net power is delivered to a capacitor.
A real capacitor, with a finite ESR, will absorb a tiny bit of power, but that doesn't change the big picture.
Efficiency of a device is defined as the ratio of useful power out/power in. Since no power is delivered to a capacitor on the output of a transformer, the power out is zero, and the ratio (power out)/(power in) is zero.