Think of the rev range required for a propeller to be efficient.
Think of the rev range required for a car engine to be efficient.
You might well say, "That can easily be solved by a gearbox". (More weight)
So you try it anyway, and find the engine destroys the gearbox. Why? No flywheel to damp out the "pulses" of power as each cylinder fires.
So you add a flywheel between the engine and gearbox, just like on a car (More weight) to get it reliable, but find you need a bigger aircraft to lift it all.
Which of course requires a bigger engine...