Actually...

Not everyone has a 6 pack.

It's well known that the anatomy of the human body varies from person to person - sometimes substantially. It's not different for the shape, origins and insertions of the muscles.

I've always had a very hard time seeing my 6 pack. I have to be at a very low bodyfat and dehydrated to be able to see it well. The reason is because of the way it is shaped.

There is a line that goes down the middle of the abs called the linea alba. Mine is pretty prominent and visible at higher bodyfat. However, the horizontal lines that make the 6 segments of a 6 pack are very shallow and uneven making a 6 pack hard to see.

Here is a professional bodybuilder with no tendinous inscriptions or even a linea alba!

http://www.musculardevelopment.com/browse/index.php?mode=browse&id=1707

Contrast that with another pro who has very deep tendinous inscriptions.

http://www.musculardevelopment.com/browse/index.php?mode=browse&id=1857