“So why hasn’t the vaunted Ukrainian offensive (in Kherson) succeeded?”
One of Clauswitz’s principles of war, is Economy of Force.
Try to achieve objectives at the lowest cost to your own forces.
Those trapped Russian forces, not equipped with Winter clothes, can be starved of supply and gradually stripped of their heavy weapons by HIMARS and drones. It is like shooting fish in a barrel - no need to sacrifice a lot of your own troops in a hasty frontal assault on a prepared and entrenched enemy.
When they are cold and hungry enough, they can shoot their officers and get to go home.
No. In warfare you either destroy an exposed force or you give it the opportunity to somehow withdraw or counterattack. The Ukrainians were not able to destroy a Russian convoy that stretched for nine miles and sat motionless for two weeks earlier in the war. The tactical opportunity at Kherson rarely occurs in war. Competent commanders would take advantage of such an opportunity.