Why doesn't a swing make a full revolution on a swingset in a park? I'm familiar with the concept of swinging of a swing in a park, but I'm confused why it doesn't it swing at a complete circle around the center bar?
 A: You mean go all the way around? It could if you had enough force to overcome gravity and like a tether ball swing all the way although most humans do not have the strength to apply the force needed to push another or them selves to a full revolution around the bar of a swing with out a jerk, but if the chain was replaced with a solid bar to prevent jerking in mid stride it could be done.
 

A: why it doesn't it swing fully ?
I think the question does point to the fact that swings which are usually available in the park does not provide free swinging , low amplitude of the swing and needs constant  pushing.
All the above is related to energy dissipation of the initial potential energy provided to the swing- and the dissipation is at the hinges provided at the support point from where it is hung .
If the load is heavier the frictional force operating at the hinge becomes substantial and a damping /slowly decreasing amplitude is observed. 
As the park swing is made for children its in a way safe also as larger amplitude may lead to accidents by falling off from the swing as braces to hold the children is not normally provided.
the swing made out with iron chains is far from an ideal simple pendulum hung from a rigid support.
A: The weight of the person on the swing
 is causing gravity to pull them down
you could swing in a full circle if you
had enough force
