Because in a GDD you are supposed to detail what stuff are on the screen and what you can do.
So if I want a button I have to mention there's a button.. what it's called... where on the screen it is.. what happens if you press it /hover mouse over it/hold it down etc... and why wouldnt the size of button also be included?
If I am making a fantasy RPG that has a warrior character, why wouldn't I include the exact size of every sprite, animation and artwork for that character? Why wouldn't I include the exact color code for every part of the character, including the color of every piece of item, equipment, weapon, accessory the character will possibly ever use? In fact, why wouldn't every single feature, change or addition to the game for the next 10 years be stated in advance?
1) Because its too much work. Some things are better done "on the fly". E.g. I'll decide the size of the button when I actually make the button.
2) It is hard to decide beforehand. There is really no way to fine tune certain details until all the pieces are there. E.g. I don't know the exact location of the "create game" button in my game lobby until I see the interface, all the other buttons, get feedback from players and see what the best location for it is.
3) Because I want my artist to have creative freedom. When I pay an artist, I am paying for his creativity and design skills. If I am doing everything myself, I might as well do the art myself and save the money.
4) Because I am not an artist. I can draw, but whatever I produce will be inferior to those from the professional artists that I am hiring.
5) Because it might depend on my technology. True story: I love JPEG. But the game engine I am using only supports PNG and so I have to use PNG.
I suppose we can both agree that some
details should be handled later.