abs are a muscle. training them more than say 2x a week is counter productive. these are muscles too and need the same kind of recovery. when training abs, you must use weights, not reps. you use your abs constantly throughout the day to lift up your own body weight. so what difference does it make if you do a 50 more reps with the same weight. you must use weights to build the muscles. this is the only way to get nice muscular block like abs.
How can you do them everyday even if you wanted too. Try mixing exercises up constantly so your body doesn't get use to the exercises. You should be sore after you work your stomach.
train them 3x a week. That's what I do, and it seems to work (at least for me). 7x week is going to leave them overtrained and sore and you'll never get them to grow. Remember, muscles grow when you rest, not in the gym! That's when you damage them!
Abs are NOT like any other muscles...your abs oxygenate more easily, highly vascularized, making for faster recovery, ability to do work for a longer time...so you can train them more than other muscle groups...same goes for calves, and some other muscles I can't think of right now.
If you want scientific evidence of this...I can give it to you when I go back to school, I'm on spring break right now.