Because the rain in most of Brazil depend on the Amazon. If the forest is gone, we will have desertification in the most populous regions of Brazil. The southeast is already facing droughts periodically, and it will get worse with more deforestation. With less rain we will produce less food, which means not only that food will become more expensive in Brazil (and a lot of people will face hunger), but we also depend on the agricultural output to avoid a trade deficit. Less rain means less food exports which means less dollars getting into Brazil, which will, among other things, turn Brazil into a hotter Argentine.
https://piaui.uol.com.br/web/six-conclusions-about-model-of-...
No seriously. There's a flying river, kilometers high in the sky, that goes from the Amazon to various Brazilian regions. It transports more water than the Amazon river itself (which is already the river with the highest discharge of the world). The water source is transpiration from Amazonian trees. Without the trees, the flying river is gone