There's more. In order to introduce the fuel to the cylinder (and I'm sticking with four strokes, because two-stroke technology is for lawnmowers) some sort of valve is needed. That (usually) has to go up and down as well, as does the other valve necessary to get rid of the burnt mixture. These valves have to be driven by another shaft of some sort, so the quest to resolve the back and forth piston movement into something rotary is compromised by the need to turn some of it back into reciprocating motion in order to make the thing work in the first place. And this is only the beginning.