Because QBs have the ability to score so much more than any other player on the field (see Culpepper, McNabb), and that's the way a lot of leagues even things out. I think it's pretty ridiculous myself, and it leads to super RB heavy leagues (RBs are already more valuable because of scarcity). So hopefully you have a good RB corps, or at least a QB capable of putting up 4 or 5 TDs and 350 yds a week.
"When I was a kid I thought I was going to be the starting centerfielder for the Boston Red Sox. Life sucks, get a f___ing helmet." --Dennis Leary