You are right in saying that there is a high rich-poor divide here. But consider this: unlike the countries you mention, Argentina freely accepts all immigrants from Mercosur countries, no questions asked. Once here, they have access to free medical care, free schools, and free universities.
The shacks outside Retiro are in the Villa 31. My cousin is an architect working for CABA, and five years ago surveyed the Villa to determine the feasibility of urbanizing it. To his surprise, he found that 60% of the residents came from neighboring countries, mostly from Paraguay.