Why does the auto manufactures give the HP at the fly wheel when really it's at the wheel what counts?

Why does the auto manufactures give the HP at the fly wheel when really it's at the wheel what counts?

Published HP #s are meant to give the power of the car’s power plant (i.e. engine). Once the car's assembled hooking up the average engine to a true engine dyno is a bunch of work. The engine needs to be physically removed from the engine bay for this. A chassis (ergo at the wheels) dyno is much easier to use for most applications. It allows for tuning and apples to applies comparisons (particularly before mods/after).

Road race engines are often tuned on an engine dyno where more elaborate and intricate calibrations are needed/possible. This is especially true for race engines running after market race oriented computer systems in leu of OEM (such as MOTEC).