And adding a rider on top of a bicycle does make it more unstable, but the amazing flexibility of the human brain allows us to transform that instability into stability, kind of like how modern fighter jets are intentionally unstable to make them more manoeuvrable.
Bicycles are a marvel of physics and biology working together.
It is totally unlike the intentional instability of a modern jet fighter.
the geometry of the ride type of a bicycle or motorcycle is really interesting 
Stuff on the road will only knock the bike over if it stops the motion of the bike first. The need for motive power was already acknowledged.