Answer: Because most rivers flow through dense, sparsely populated forest regions or have falls on the highlands, away from the populated east coast.
Most of Brazil’s population and economic activity is concentrated in the eastern coastal region, while the large rivers like the Amazon flow through dense equatorial forests and swampy land where population is very small. Rivers on the Brazilian Highlands flow over steep slopes with waterfalls and rapids, which are not suitable for navigation. Hence, apart from commercial navigation on the Amazon and the Parana, waterways are not much developed, and roadways carry more than half of the transport.