Why Do Abs Exercises Have So Many Reps?
Short Answer: To feel "The Burn"
Many people erroneously feel that most abs exercises are cardio exercises. As a result, they try to perform as many reps in as short a time frame as possible.
First, because the abdominal muscles perform many functions,they're capable of being stressed longer than most muscles. As a result, the burn isn't usually felt until after a lot of repetitions.
Should Abs Exercises Have So Many Reps?
Abs exercises should be performed in a slow, methodical fashion; there should be a pause at the top and bottom of the movement. Equally important, the exercise must target the specific muscles being exercised.
When performed with these considerations, "the burn" will be felt in as low reps as between 10 and 15.
Another Note Worth Considering
No one single abs exercise target every part of the abdominal muscles; you'll need to perform different exercises that target each segment of the abdominal muscles.
Without a low body fat percentage, abs exercises don't mean squat aesthetically. The muscles might be strong and puffy; however, without a low body fat percentage, you won't be able to show them off (this could be one of the reasons people use abs exercises as cardio).