The simple answer is because they are trying to make lots of BHP from a small engine.

BHP = Torque * RPM / 5252


So only TWO possible routes to more power:
1) more torque
2) more rpm

They are already fairly good for torque for the size engine that they are to be fair (despite what people say to the contary), so the only option for big power is to design them to rev a lot harder.

This is done by short stroke's to keep piston speeds down, and a lot of valve area per CC of displacement, along with of course the required mechanical strength of components.