We got bombed by Japan which is an obvious reason to go and stop the Japanese from trying to invade the mainland.

This is incorrect. Wartime hysterics aside, most American policymakers recognized that Japan did not pose an existential threat to the mainland United States. Japan barely had enough shipping to seize SE Asia, the Philippines, New Guinea, and the Solomons. While there were occasional worries among American military chiefs that Japan could be more powerful than it appeared, their assessments of Japan's capabilities were more or less grounded in reality. The American negotiators with Japanese envoys in the run up to the war said as much to their counterparts between 1940 and 1941. The Joint Army and Navy Board had been wargaming a conflict in the Pacific since the 1920s and the plan of attack against Japan, War Plan Orange, anticipated that the Japanese at best could strike out at the periphery of American power (ie Alaska or Hawaii), but could not directly threaten the US mainland. The Orange plan anticipated an organized, central drive into the Pacific.

Germany was a slightly different kettle of fish for American planners. For one thing, the German conquest of Western Europe shocked many observers within the American government and beyond. Not only was the scale of the conquest unprecedented, so was its speed. France which had resisted and triumphed over German invasion twenty years before, fell in a matter of weeks despite a large and modern army. Hindsight renders Operation Barbarossa a clear failure, but things were less obvious from the standpoint of December 1941. Germany had captured a large swath of the USSR's industrial heartland, captured vast numbers of Soviet soldiers in encirclement battles, and the ability of Stalin's government to make good these losses was an open question. Therefore, "Europe First" made much more sense from the Americans' strategic priorities. Germany had demonstrated a much more viable emerging threat. Germany might not have possessed the forces in 1941 to cross the Atlantic, but leaving it in uncontested control of the European continent was not a means to ensure this state of affairs to continue.

Nonetheless, for all of the prioritization of the ETO, it bears stressing that the US did not merely stay on the defensive in the Pacific. The USN was actually incredibly aggressive in the context of Japanese air and naval power throughout the first half of 1942. The ETO may have gotten more manpower, but the Pacific still received its share of resources that allowed a series of offensive drives throughout 1943 and 1944. Such was the industrial power of the US that it could support major military offensives on both hemispheres.