Because that would suggest that ALL amplifiers with different operations classes, types of FET and wiring singel ended/Push pull. Gives the SAME constant and relationships so only from wattage and resistance you can get the I and V.This is correct even though you don't believe it. If a given amp makes a certain power into a certain load, then it will be making exactly the same voltage and current as any other amplifier that can do the same thing.
What might be tripping you up here is output impedance plays a role. If the amp has a higher output impedance, more of its output power will be dissipated in the output section itself rather than the load when presented with a low impedance load. Since no loudspeaker is a flat impedance from top to bottom, this means that different amps can sound different, even though at certain impedances of the speaker they might all make the same power.
To further muddy the waters, not all speakers are meant to be driven by amps with a super low output impedance, while others are. For more on this see:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php