Because
Rage's install size will be a whopping 21-22gb, and no, there is no partial install option. Also, without an install there will be latency, so I guess don't install it at your own risk. According to
beefjack.com, John Carmack himself revealed all this during QuakeCon, so this info is straight from the horse's mouth. I guess it's time for some of you to buy bigger hard drives or execute a cull of epic proportions.
Rage is supposed to be out on October 7th, and I don't care how big the install is,
Rage still looks amazing.