Why is the power draw measured in milliamps, wouldn't watts be more appropriate? The voltage of the battery drops as it discharges, so 500mA when it's fully cha

Why is the power draw measured in milliamps, wouldn't watts be more appropriate? The voltage of the battery drops as it discharges, so 500mA when it's fully charged isn't the same as 500mA when it's nearly empty. Unless that is taken after the voltage has been regulated, but in that case again why are we measuring the capacity in amp-hours instead of watt-hours?