Seems like a very strange question, from the Japanese point of view (at the time) they were on par with the power of Western nations and were surrounded by 3rd world, much weaker nations abundant in resources, so from their perspective, why not?

It is a tricky proposition in history to look at the long-term outcomes of decisions and judge the decision-makers accordingly. In this case, Japan's interventions in China long predated Pearl Harbor and even the outbreak of the Second Sino-Japanese War in 1937. The first direct control over Japan exerted over mainland China actually predated its takeover of Korea. The Japanese received the Kwantung Leased Territory on the Liaodong Peninsula in 1898 as a result of the First Sino-Japanese War. Although a consortium of Western powers forced Japan to return this concession, Japan received the leased territory back in the Treaty of Portsmouth. The Kwantung area became a center for Japanese financial and political penetration of China. As a free trade port, Japanese products could flow into northern China and benefit Japanese industrial interests. The semi-privately owned South Manchuria Railway Company grew out this territory by virtue of its close connections with the Japanese government (its first president was Goto Shimpei and it was organized by two prominent IJA generals) and branched out into a major investor in Chinese infrastructure and extractive industries.

The problem for Japan's continued involvement in China and why efforts to put a break on expansion is that Japanese involvement within China benefited multiple interests within Japan. The IJA saw Japanese military expansion into China as fulfilling multiple interrelated goals. Firstly, operations in China would give the IJA a mission and expand the security zone Japan had created by its annexation of Korea. The IJA also saw increasing involvement in Chinese affairs as a means to secure its budget versus the IJN and ensure that the Taisho era bureaucrats would not slash army appropriations in the 1920s. The existence and maintenance of a large military force in Liaodung, the Kwantung Army, became an end onto itself for the IJA. Japanese political liberals like Kato Takaaki, Japan's foreign minister in WWI and later a PM in the 1920s, conceived of Japanese imperial initiatives on China like the 21 Demands as part of a larger process that would modernize China under Japanese tutelage. The 21 Demands, which were an ultimatum given to China in 1915 which would hand control over much of China's national security to Japan and expand Japanese control over the Chinese economy, had a certain logic from Kato's liberal perspective. The Chinese reform movement had certainly stalled out since the 1911 Revolution and Kato presented the 21 Demands as a means to reenergize China by letting Japan reorder Chinese affairs. Kato also hoped that the 21 Demands would outmaneuver the IJA's own political faction within the government headed by Yamagata Aritomo which wanted a more bellicose expansion and decouple Japan from the international system. Japan's financial sectors initially favored the liberal approach to empire favored by Kato as it fit in with the larger attempts to forge a Yen-bloc within East Asia in which the Japanese Yen became the dominant currency for East Asian trade. Treaties like the 21 Demands were much more stable platforms which to accomplish this project as they fostered the economic infrastructure and connections between financial institutions.

These major factions within Japan were seldom stable and the and political realignments and shifting strategies characterized the Japanese imperial policy of the late 1920s and 30s. The IJA's vision for a Japanese-dominated China grew more ambitious in both scale and complexity. Military leaders within the Kwantung Army like Ishiwara Kanji began to perceive Japan's interventions within China as part of a larger project that would make Japan a pan-Asian hegemon. The Kwantung Army became an incubator for radical ideas within the IJA as a whole as a leader like Tojo Hideki advocated for a Showa Restoration that put the military on control over the government. Finance shifted support for the IJA's project to create a Chinese puppet state in Manchuria because both the Great Depression destabilized the gold standard and wider economic reverberations from the Depression made Manchuria a more stable investment. The IJN, which was a traditional brake on the IJA and favored southern expansion, became more mired in the IJA's tentative military steps into China as it was afraid of losing the budget battles within Tokyo. Japanese liberal politics also became preoccupied with issues of domestic stability. Although the Depression did not impact Japan as harshly as other industrial states, the fear that it could destroy the Japanese economy became quite animating for these politicians. State investment within Manchuria became a way to strengthen the Japanese economy and reemphasizing the imperial project in propaganda was a way to animate the population. This also served the purpose of mollifying radical opinion in the IJA by allowing them access into the Japanese government.

Ultimately, one of the basic problems of Japan's Chinese policies remained unresolved: the attitude of the Chinese towards Japanese imperialism. Imperialism seldom concerns itself with the public opinion of the colonized and Japanese imperialism was not the exception in this regard. Although involvement in China suited the needs of various Japanese political factions and varied over time in tone and severity, these initiatives ran roughshod over issues Chinese national sovereignty. Few mainstream Chinese intellectuals and politicians were prepared to accept Japanese control. The persistence of China's protests made it hard for Japan to present its imperial gains in China as a fait accompli and made Japan look more like a rogue state in the eyes of world public opinion.

Sources

Dickinson, Frederick R. War and National Reinvention: Japan in the Great War, 1914-1919. Cambridge, Mass: Harvard University Asia Center, 1999.

Schiltz, Michael. The Money Doctors from Japan: finance, imperialism, and the building of the yen bloc, 1895–1937. Cambridge, MA: Harvard University Press, 2012.

Young, Louise. Japan's Total Empire Manchuria and the Culture of Wartime Imperialism. Berkeley: University of California Press, 1998.