The same reason you actually have to fly on an airline ticket to get mileage credit.
If you didn't have to actually show up at the hotel or take your flights, then it would be too easy to qualify for promos, status, etc.
Not meaning to be argumentative here, but Hyatt does not prohibit this pratice. With the Hyatt FFN promo that can lead to some very cheap elite qualifiing stays and free nights to boot.