Why does GPU consume so much power?
A GPU is basically a LOT of simplified CPUs in parallel. Each of them is not as capable and flexible as a real CPU, but there are thousands of them to give this massive parallel computing performance.
But this also means that it takes many billions of transistors to build a modern GPU. And for logic chips we use FETs, so with every clock cycle all the billions of gate capacitances have to be charged and discharged. This is where the large amount of power is going.