So, why don't they ask US to leave? Or the Europeans?
Much has changed in 75 years.
Koreans made their preferences for freedom and independence clear in 1945, and white supremacists on the opposite side of the Pacific prevented popular sovereignty from occurring:
South Korea - South Korea Under United States Occupation, 1945-48
"Lieutenant General John R. Hodge, commander of the United States occupation forces in Korea, was obliged to work under a severe handicap--a mission of maintaining peace and order until the international conflict over Korea was resolved.
"Possessing very limited resources, Hodge was expected to pursue the 'ultimate objective' of fostering 'conditions which would bring about the establishment of a free and independent nation.'
"General Hodge had to contend with hostile Korean political groups. Before United States forces had landed in Korea in September 1945, the Koreans had established self-governing bodies, or people's committees.
"The leaders of these committees had organized the Central People's Committee, which proclaimed the establishment of the 'Korean People's Republic' on September 6, 1945. Exiles, abroad, mainly in China, had organized the "Korean Provisional Government" in Shanghai as early as 1919 and had sustained a skeletal organization in other parts of China until 1945.
"The United States recognized neither the republic nor the provisional government."