We don't have law of conservation of voltages right? Only the power should be conserved. In that case why can't the output current reduce and increase the voltage so that the energy is still conserved?
Unless you have an inductance or capacitance to store energy and supply the extra voltage (such as a switching regular or charge pump), an amplifier can only deliver an output voltage equal to the the supply voltage. Think of a standard amp as a variable resistor connected between the power supply and the load. Even if you reduce the resistance to zero, the maximum voltage can still be no greater than the supply voltage.