The hip is a ball in a socket and the knee is a hinge
The top of the femur is a ball that sits in a deep cup on the pelvis. That ball-and-socket lets the thigh swing forward, back, out, and around. Stability comes from the depth of the cup and from the muscles and ligaments around it.
The knee is built more like a hinge. It mainly bends and straightens. When it is bent it can also twist a little, which a door hinge never does, but the big motion is still one plane.
Two neighboring joints, two jobs. One aims the leg. The other folds it.
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