Why GPU exists?
GPU stands for Graphics Processing Unit, and it was originally built for processing/rendering graphics.
Let's think about what that demands. A 1080p display has over 2 million pixels each need a colour value around 60 times a second for motion to look smooth.
That colour needs to be calculated based off how much light hits a surface, what texture is mapped onto it, what angle the camera is viewing it from, whether something is casting a shadow on it etc.
One pixels colour computation is completely independent of the other, meaning pixel (400, 200)'s colour computation doesn't need to know anything about pixel (1100, 800)'s colour computation.
This is not just a small implementation detail, it is a single fact the entire chip is built around.
Because once you know you have 2 million independent calculations to finish 60 times a second, the right move is not to build one very fast calculator.
It is to build a lot of tiny calculators and run them all at once.
The GPU wasn't built as a smaller, less powerful CPU. It was not built to answer the question "how do I finish one task fast?".
It was built to answer,
"how do I finish an enormous set of identical tasks at the same time?".
So every design choice you will see downstream, thousands of small cores, warps, and SIMT is there to complete that answer.