That's what confuses the moto-geeks about Harley's. They were designed and built in the Great Mid-West, the land of the tractor. Since the first Harley's were built in a shed, the peripheral components available to HD, such as ignition, oiling and lights were essentially from vendors that supplied tractor companies such as Deer and Massy Ferguson. Many engineers at HD had tractor company experience before working for HD. A Harley is not British "Bike" or Japanese "Rice Rocket", it's an American "implement".
The Italian company Moto Guzzi actually did start out making farm machinery. Their trademark 90-degree v-twin with a longitudinal crank was originally designed to power some sort 3-wheeled vehicle, I think.