EDIT: Ok, I think I see what you mean now. I believe your question is about what is the appeal in doing this stuff. It's pretty much the same reason why someone would build a chair or a table from scratch versus buying one at IKEA and building it. The former gets the enjoyment from all of the journey from a piece of wood to the finished chair and from building exactly the chair that they want and the latter is probably more focused on the chair itself. END_OF_EDIT
As for the rest, I believe that some answers are already in the post and in the comments that I and other people made.
I may have misunderstood what you meant and if that is the case let's restart from scratch:
Do you want to program a raspberry pi at the hardware register level, without any APIs ?
What EXACTLY needs to be reverse engineered to "make old school games" or "fully functional operating system"?
It's an ARM and the GPU has some API.