Brazil has something like 50% of south America's population

It also has something like 50% of South America's land, and an average population density almost identical to that of the continent as a whole.

You can pull up relevant population statistics from the UN here. For 2015, the UN gives Brazil a population density of 24.9 people per square kilometer, compared to 24 for South America as a whole. By contrast, Ecuador comes in at 65 and Suriname at 3.35.

I'm sure there are many valid questions to be asked about the differences between Brazil and the rest of South America, but I think this particular question is predicated on a faulty assumption.