You can buy a $4,000 6th man after the draft for probably $10,000. But if one of those unscouted guys turns out to be a superstar or a MVP then he could be worth $100,000-$500,000 even at $3,500 salary, and someone who could eventually become your best player ever. Why risk missing out on a shot (very small) at getting someone game changing to get someone who is certainly not!