WHY is the power of machines measured in horsepower?

Since the first steam engine was invented, it was necessary to determine its cost-effectiveness as a replacement for horses, and therefore how much money to pay for the machine. Some work was performed with a steam engine, then with horses, finding how many horses could do the same work - so we got power in horsepower.

The short term power of a horse is much more than 1 hp, the long term power of 1 hp can provide not every horse (it was beneficial to the buyer of the steam engine, so the horses were selected hardy enough).

Later on hp was abandoned as not related to other physical quantities directly, without coefficients, but for figurativeness and by inertia of thinking, technical devices, the power of which a person must directly feel (car, airplane, ship) left for use the expression of capacity in horsepower.