horsepower is a fake number generated by measuring torque and using a constant number.

It’s just a unit of power. You can calculate watts from force x speed, too, if you use metric units. Yeah, you can shift the whole thing to a rotational system with torque and rotational velocity (and then derive the linear equivalent force and speed from those units as the radius of the circle cancel out).

Only applicable to gasoline engines.

Power is power. By using similar units it makes it easier to compare different vehicles. If everyone wants to convert to watts, we can, but we’d still be making those comparisons.