Why is the Amazon River not used for irrigation or hydroelectric power generation?

In "flood" season, the Amazon can reach an average of 40 kilometres wide. Anything that would be irrigated by it doesget irrigated by it, and any hydroelectric station would be simply bypassed.
Back in the days when Africa and South America were joined, the Amazon was part of the watershed of an African river.

When the Atlantic split, the modern Amazon developed, but because of the reversed drainage, it still has very little fall over its length. [Apart from close to the Andes.]

It has a HUGE area of river basin, and the flood plains are tens of metres above the river.

And because the climate is moist, there is less need for irrigation.

Irrigation is usually only useful where there is intensive cultivation, generally forced by population pressures.