Why were Ethiopia and Liberia the only two countries in Africa to not be colonized?

I would argue that, in fact, Ethiopia and Liberia were actually colonized.

Liberia was founded as a state by African-American settler colonists, whose descendents became the ruling elite. While this effort was undertaken by a private organization, the American Colonization Society, it was supported by the U.S. government. Much like de jure independent states in Central America and the Caribbean, Liberia was a de facto resource colony of the United States. Rubber was one particular commodity which U.S. corporations extracted from Liberia (and still do today).

Ethiopia has a history of being perhaps the strongest imperial or national state in pre-colonial sub-Saharan Africa. During the Scramble for Africa, Ethiopia repelled an Italian invasion in 1895-1896, preserving its independence. However, much like other "independent states" like Iran and China, European countries still tried to dominate it economically. In particular, the French exerted influence by constructing and owning a railroad that connected the Ethiopian capital of Addis Ababa to its colony in French Somaliland (Djibouti). Eventually, Ethiopia lost its independence in the Italian invasion of 1935-1936 which was also a move against French and British interests - Italy went on to annex French Somaliland and British Somaliland after the outbreak of World War II. But the war also spelled an early end to Italian colonial rule in Ethiopia, as the British invaded to liberate Ethiopia in 1941.