the guys you see on the front of the mens health mags (or whatever other mags feature models with six packs) have been startved for a few days before the photo shoot. in addition to hitting the gym A LOT to develop their abdominal muscles, they need to shift the blubber covering it so you can see it.
compare how a boxer looks midway during training with how he looks at weigh-in, when he's starved to squeeze into the weight category. They'll then gorge on carbs for the next 24 hours or so to get energy (and a couple pounds in weight) for the fight.
doesn't matter how developed the muscles are if you've got even a little bit of fat covering it, then it won't look as defined as the models on the mags do. They go through cycles of building and cutting (where they hugely reduce their carb intake). during the cutting they're not particularly strong because they've got no energy, the carb reduction means they lose a load of retained water, and look really trim, but it's not a very functional state to be in.
as mentioned above, there's also genetics to consider, if the frame you inherited is just not naturally lean it'll be difficult to look cut. You can do it, but you'll be swimming upstream.