MacArthur felt he could not sit still, T. R. Fehrenbach explains (in This Kind of War), and let his troops be tied up for the winter:
The CCF plan might be to make Korea a permanent running sore, and to tie up more than a hundred thousand U.S. ground troops indefinitely. With winter already howling down out of one of the coldest spots on earth, he had to retreat or attack. He attacked.
[...]
“I believe that with my air power, now unrestricted so far as Korea is concerned…I can deny reinforcements coming across the Yalu in sufficient strength to prevent the destruction of those forces now arrayed against me in North Korea.”
[...]
“The giant U.N. pincer moves according to schedule today. The air forces, in full strength, completely interdicted the rear areas, and an air reconnaissance behind the enemy line, and along the entire length of the Yalu River border, showed little sign of hostile military activity.”
[...]
Whatever the weaknesses of his ground forces, whatever their difficult and exposed positions, U.N. mastery of the skies was complete, and air would be the decisive arm. It was a typically American viewpoint.