Why are wireless chargers inefficient?
Because it's a transformer with poor coupling between the primary and secondary, because it's an air core so it permeability is not good (I.e how well magnetic field can pass through) and the turns on the secondary are few. So if you have poor coupling and low secondary inductance, how do you get a few watts in the secondary? Pump lots of power in the primary of which alot does not pass across. This energy is therefore wasted thus lower efficiency. Fast chargers scale this so often wireless fast chargers have fans or large heatsink because the primary energy is much higher.