I also have this problem when I try to do high rep crunches. I asked myself why I was doing so many reps for abs when I wouldn’t normally do such high reps for other body parts?
I abandoned the high rep training, started rotating through movements like kneeling cable crunches, hanging pikes, weighted hanging knee raises, and cable pull-ins for sets of 6-8 three times a week. The extra hypertrophy I got from training this way actually gives me more definition in my abs, and I’m stronger on my core lifts. I do some rotational movements (full contact twists, woodchops) for slightly higher reps and stabilization stuff (planks, core statics, etc.) as well.
In short, bunches of crunches didn’t do shit for me so I dropped 'em.