How come all digital cameras use this system for storing photos?

How come all digital cameras use this system for storing photos?

Root directory: DCIM (and sometimes MISC)
Inside DCIM: 100xxxxx (Five letter word like NIKON, CANON, PENTX, CASIO….)

What is the “Misc” folder for?

What does “DCIM” stand for?

Why exactly five digits after “100″?

Who set this standard for all digital cameras?

Thanks for answering this dumb question!