First of all, the idea of having or obtaining "good abs" as some sort of accomplishment or measurement of athleticism is a myth. Everyone has abs. Those whose abs are most visible are the ones with the least amount of body fat on their stomachs. This is why it is not only common, but pretty easy for a ten year old to have visible abs. You can see the abs on most skinny kids.
Some people are genetically predisposed to having very low body weight and among these, there are those who are lucky to keep trim waste lines even when they do gain weight in other areas.
Young men tend to have less fat on their stomachs than women and far more high school boys can boast of "6 packs" or "8 packs" than high school girls, usually without trying to obtain them. I suppose when a girl claims she's attracted to nice abs, she's admitting she is attracted to men who, like previous posters have stated, are fit and thin.