The reason why is we can add part by part to see what it is. Also avoids the problems of case shorts.
That is a common gaffe is to put all the standoffs in the case. Where there should be any.
Cardboard building is best the first time out. Start small.
I just built a Gaming PC and I am trying to start it up. There is sufficient power in that the fans and LED lights turn on when I power up the PC. But, I still see no display on my monitor. When I first turn it on, the fans on my graphics card spin for a little, then stop. I have a GeForce GTX 1050 Windforce OC 2G graphics card. I don't know why I can't see anything!