Pistons move very quickly, and faster and faster as engine speed (RPM) increases. Eventually, getting the piston to change direction becomes not only a parasiti

Pistons move very quickly, and faster and faster as engine speed (RPM) increases. Eventually, getting the piston to change direction becomes not only a parasitic loss of power on its own, but introduces a lot of design challenges around the strength and design of the crankshaft bearings, connecting rods, and piston retention pin. Furthermore, the crankshaft should balance the weight of the pistons, so losing weight from the pistons also loses weight from the crankshaft. Reducing the mass of the pistons often adds more headroom in these components and enables the engine to reach a higher maximum operating RPM (redline), and since horsepower is a function of torque x speed (RPM), increasing redline often directly increases peak output.