because brazil is the biggest economy in south america by gdp beating out mexico and having a nominal gdp almost four times greater that of argentina (3rd place on the continent). also brazil has the biggest population on the continent (four times that of the runner up columbia). brazil is absolutely the powerhouse of south america. even if argentina gets its shit together it'll still be a runner up to brazil. the fact that you even ask this question in the first place means you are either young or retarded or both.
United States