Why is it that abs can be worked every day?

I don't think anyone should work any muscle everyday. When you work muscles you fatigue them or cause tissue to be ripped apart and broken down and they need time to heal and get stronger. If you work a muscle that is not fully recovered you risk a possible injury or further tissue breakdown which will only take longer to heal and limit your body's resources for growth and development.

Many people think that small areas like abs, forearms, and calves can be trained everyday. Now I do believe that these small areas may recover faster than some of our larger ones because of their common use in day to day activities they seem to contain more slow twitch muscle fibers. Slow twitch muscle fibers are more efficient at using oxygen and experiences less lactic acid buildup than larger muscles. In my practice working abs once or twice a week has been sufficient.

More now than ever I lean toward using core stability exercises and isometric holds such as the plank and swiss ball for complete ab development rather than the generic ab movements you see people do. (ie. Crunches, leg raises.)

Remember there is more to abs then what we all strive to see visually, the rectus abdominals or that more commonly known as a 'sixpack' can be a major hindrance to overall strength, posture and athletic performance when overdeveloped.