Because if you dont fool it to thinking there is a load on it it will turn on and stop. 

Converting Computer ATX Power Supply to Lab Bench Power Supply

El Cheapo 20A Lab Power Supply from ATX PC Supply

Again, it all boils down to not wanting to spend $14 when I have everything I need minus a $3 Power Resistor.  I can buy a few items I need for modeling wise for $14.