Some US policymakers did - arguing for a broadening of the war got McArthur fired and he was not without his sympathizers in Congress.

However, Korea should be thought of as the first real proxy war. China had the implicit backing of the USSR for their intervention and thus an invasion or significant military attacks on China would have likely meant a global war breaking out, something that few in Washington were particularly excited about.

Beyond that, the US rationale for war and the resulting UN peacekeeping mission was that the North had broken the armistice on the Korean peninsula. Expanding the war to China would have also broken the US-led coalition.

Those in favor of broadening the war were apparently gambling that the Soviets would not respond to an attack on China while the Truman administration was unwilling to take such a risk and also risk shattering the international coalition that they'd built to defend South Korea.