WHY I want to learn about Graphic API

Hi, I am 3rd year computer engineering student, doing mostly unity developer and a bit of shader as a hobby. I also have fundamental in c++ and c.

I almost graduate but I still don't know what field should I work in, unity dev? Technical Artist? or Graphic programmer?

Therefore, I want to try on learning graphic API like openGL, vulkan, or WebGPU ( I still don't know what to choose, that's another problem in the futer LUL ), but more important question is why I want to learn or build something from graphic API. what is the problem with current general-purpose engine that make need to make a custom graphic engine. for example, a custom game engine is made because there are specific problem like a weird physic in noita.

But what about custom graphic engine? what is the reason to build a custom graphic engine and what industry need from graphic programmer. Thanks!

I watch this video from acerola but I still want to know more in depth. https://www.youtube.com/watch?v=O-2viBhLTqI&pp=ygUSZ3JhcGhpYyBwcm9ncmFtbWVy