Though it hasn't been posted yet..and with great hesitation for fear of sounding judgmental...I'll go ahead and ask why it's at all important to have one?
Really, it isn't about core strength. It's about 'showing' the muscle. As someone posted, you have to drain yourself down to get to that level, and I still maintain that not everyone's body can even handle such a low level of body fat. I dunno.
Why is it important? Ego? Impressing others? LOL. Seriously, I'm with you. I work out 3 times a week, I make an effort to do cardio, which consists of jogging on a treadmill. I walk daily, a few miles, with my dog, and there are times, when I'll wear a weighted vest, and on days when the weather is bad, I'll wear it inside and go up and down my stairs in the condo. I try to watch what I eat, but of course, I can't always be good..lol. So, with all that, no, I dont have a 6 pack, nor do I care if I ever have one.