Just nitpicking at your semantics a bit here, but it isn't truly your 'abs' that get used during the big lifts anyway.
What most people refer to as the abs, the rectus abdominis, i.e. classic '6-pack' muscle, is primarily a locomotive muscle responsible for trunk flexion, and also resisting the opposite movement, aka trunk hyper-extension. From a bio-mechanical standpoint, none of these happen to a significant degree while squatting/dead-lifting.
It is your lower back muscle groups that get trained, because odds are your core is resisting gravity trying to force it to fall forward, as opposed to falling backwards (which would be what would properly hit the abs, hence why Overhead Press, where your center of gravity is so radically shifted along a vertical plane, as well as front squats, are probably better for ab stimulation than squats/DLs).
So yes, squats/DLs definitely give your core quite a whipping, but Derrick is right that core =/= abs. Rectus abdominis = abs. Squatting heavy absolutely annihilates core muscles like your multifidus, transversus abdominis, pelvic floor muscles, etc, but not necessarily your rectus abdominis. Specifically training the function of the Rectus abdominis, i.e. trunk flexion, like Derrick said, also is of little discernible benefits to your main lifts, so pretty much the only reason you'd work that is if you wanted to specifically elicit hypertrophy in that region (a common enough goal, admittedly). One exception would be if you're one of those guys that naturally have an overpowering lower back, so a weak set of rectus abdominis could cause certain posture issues.
Thanks for the compliment.
