Why does amplifier power efficiency not consider the power input by the input signal?
Welcome to stack exchange Cojoye!

In a power amplifier, the flow of current is primarily from the supply to the output. So from this perspective, it might be easier to think of the supply as the "input power" and the output as the "output power".

In regards to the input signal, we should keep in mind that the input impedance to a good amplifier will be extremely large. So large that the current being drawn from the upstream input signal is negligible. No current means no power! Therefore it is usually not considered. The input should be considered as just a control signal carrying information, not a source of power.

Let me know if anything is unclear!