To be frank, because the US and Colonial Powers were the ones that Japan had reasons to fight or want to fight.

Britain and France had been very active in the industrialization of Japan during the Meji Restoration, but had also amassed large colonial empires in the resource rich South Seas along with the Dutch. Meanwhile Japan had risen from an afterthought to a major world power following its victory over Russia in 1905 and early imperial adventures in Korea, Taiwan and elsewhere. Suddenly as a potential foe relations soured between Japan and foreign powers, exemplified by the breakdown of the British-Japanese Alliance and naval cooperation. Into this then comes the rising US, a nation with major economic and security interests in the Westpac and a colonial empire centered around the Philippines. And which took a dim view of another power on an expansionist path in the region. It could easily be argued that the Pacific War as generations in the making, the earliest serious versions of War Plan ORANGE in the US for instance emerged even before WW1.

As icing on the cake then Japan was forced to accept what many felt was the insulting results of the Washington Naval Treaty. The treaty limited tonnage and ships to a rough 5:5:3 ratio with the US and Brits who argued their global commitments required the additional ships while Japan's interests were much more localized.

While we would also be remiss to not touch on the racial aspect here. A whole industry of writers and thinkers emerged in Japan arguing for the casting off of the chains of white imperialism, and the advancement of all Asian people together from under centuries of insults. Under the leadership of the great Yamato Race of course! While staggeringly hypocritical the rhetoric behind the idea of the Co-Prosperity Sphere was more than propaganda in many circles. While with ever tightening economic restrictions from the US and Europe in the midst of rearmament, and Siberia not being a very feasible option. The islands and resource centers of SE Asia began to look very appealing in order to continue their war to subjugate China. Japan had been waging a separate war with the various Chinese governments since the early 30's to secure the resources, and markets of China, to make them safe for Japanese control, while also setting up a client kingdom in Manchuria.

The only problem though was that between the Southern Resource Area and the Home Islands sat the Philippine Islands and the US garrison. And the senior leadership in Japan did not see a way that they could strike south without drawing the US in, or commit sufficient forces along supply lines to protect them. All the better to begin the war on their own terms. And the senior admiral at sea, Yamamoto had a scheme to do that with a strike to open the war on the primary USN fleet base at Pearl Harbor!

As an introduction to the origins of the war that is good for newcomers to the topic I would suggest Pacific Crucible by Ian W. Toll which is the first part of a trilogy on the War in the Pacific he is writing.

Japan was waging a separate war in the pacific throughout the 1930's. The target of their ambitions was China. As of the early 1930's, they had conquered Manchuria, but in 1937 (still 2 years prior to WWII breaking out in Europe), they renewed the conflict in the hopes of capturing more territory. This is known as the "Second Sino-Japanese War".

Initially, China had a friendly relationship with Germany, who sent military advisors and equipment to China throughout the period. However, Japanese and German diplomats eventually came to an understanding on account of a mutual rivalry with the Soviet Union, and German assistance to the Chinese ceased. Japan had historically fought conflicts with the Russian Empire, and Japan fought multiple border-skirmishes with Soviet forces throughout the 1930's. Critically, one of these skirmishes, a decisive Soviet victory, resulted in a non-aggression pact between Japan and the Soviet Union. Japan and Germany both sought cooperate against the Soviets, although that did not necessarily mean that they saw eye to eye on all issues, or that they would selflessly help each other.

The invasion of China had angered the United States in 1937, resulting in the termination of trade agreements and a general deterioration of relations. After the fall of France, the Japanese moved to occupy French Indochina, which also greatly upset the Americans. All of these factors combined with a withdrawal from the League of Nations and the signing of the Tripartite Pact with Germany caused American policymakers to embark on a policy of economic containment of Japan. A series of trade embargos were undertaken in collaboration with the British, starting with Scrap Metal (vital to Japanese construction of warships in particular but also important in general) and culminating in Oil (vital to most economic activities).

At this point, the idea of Japan entering WWII on the Allied side would be completely ludicrous. In order to repair relations with the west, Japan would have had to abandon their ambitions in China and suffer a humiliating international setback. The path they chose was not a complete shock - the idea of moving towards war with America and the United Kingdom as a means of securing their territorial ambitions in Asia - but the specific form it took as a massed air assault on Pearl Harbour was a shock, a huge one.