Because the peg is not instantaneous and doesn't try to be, that would be foolish. According to Steemdollar.com the price of a Steem Dollar is currently $0.90. What you will notice is that buying low (below $1.00) and selling high (above $1.00) is easy money. While you hold you also get interest.
Steem is volatile, market makers haven't fully engaged. The profits for market making are high right now.
We created a comic strip where @dan becomes the grammar man to Charles Bovaid :) he brings up an interesting point where bitcoin is an appcoin too.