Because shapes with circular cross-section resist internal pressure better than any other shape.
Spheres/cylinders are ideal for converting internal pressure into hoop stress -- they naturally keep their shape when subjected to radial force from pressure. All internal wall stresses are pure radial compression and pure hoop tension. There is no shear component.
Other shapes create bending stresses and deform under pressure. So cylinders are the obvious solution when you need a piston to be pushed by pressure.
But not all internal combustion engines use cylindrical pistons! Wankelengines use an oval and a Reuleaux triangle (or very similar shape anyway) to produce a rotating motion in a non-cylinder shape:
Spheres/cylinders are ideal for converting internal pressure into hoop stress -- they naturally keep their shape when subjected to radial force from pressure. All internal wall stresses are pure radial compression and pure hoop tension. There is no shear component.
Other shapes create bending stresses and deform under pressure. So cylinders are the obvious solution when you need a piston to be pushed by pressure.
But not all internal combustion engines use cylindrical pistons! Wankelengines use an oval and a Reuleaux triangle (or very similar shape anyway) to produce a rotating motion in a non-cylinder shape:
