Inefficiency at several steps.

Battery voltage to charging voltage is a step.

Charging method (wireless) adds a step.

Input voltage to battery voltage is a step.

Plus you have the extra circuitry of Wireless to power.

Nothing runs straight off 5v of USB or 3.7v (nominal) of LiPo. There's little converters for everything, every step of the way.

This is why you'll never see a 3.7-4.2v battery pack delivering 1:1 charge to a smartphone over 5v USB. I did the math a long time ago and the capacity conversion is around 74% to 84%. So if you have a 10,000 mAh battery, expect it to charge about ~8000mAh worth of your phone.

For wireless, it's even worse.