The relief of the Amazonian Lowland and the Brazilian Highlands formed on the same platform, but it is different. Why?

Both the Brazilian Highlands and the Amazonian Lowlands are formed on an ancient South American crystalline platform. The relatively flat relief of the Amazonian lowland has developed as the Amazon River, its many tributaries, high rainfall and rapidly growing vegetation have contributed to the decomposition of volcanic rocks, so the lowland is now covered with sedimentary rocks. On the Brazilian Highlands, weathering is much slower, and therefore it is higher and more uneven, hilly.