Sitting is an act of balance. In life this act is coordinated by our nervous system and performed by our postural muscles. When the right support is on the sit bones (those 2 bony parts under the bottom) and the head is freely balancing on top of the spine, it is then when you can find the right equilibrium.
Why is it then so difficult to maintain?
First, simply because our body is built to move, not to stay still for hours; second: faulty sensory appreciation. The stress, stiffness, the rhythm of life, bad habits, all of these interfere with the system to find the right natural balance.If you want you can learn how to restore it.