Your body weight is being supported by a machine in a leg-press or hack-squat, whereas a raw-squat forces you to lift your body as well. There is also the rigid, supported movement of the machine to consider, so you don't have to worry about balancing left-right-forward-back, which also take some amount of effort when lifting large weights.
There's multiple parts to this (balance/stabilization, resisting your bodyweight against gravity, etc.), but it all essentially comes down to bracing. Bracing is essentially how well you can push into something unmoving, in direct opposition of the load you're using to train the muscle.