Why do your ribs not go all the way down our abdomen?

Ribs extend down to the abdomen but do not fully enclose it:

Adult humans typically have twelve pairs of ribs: seven "true ribs" that connect directly to the sternum, three "false ribs" (including two that attach indirectly to the sternum via cartilage and one floating pair of ribs) below the sternum, and two more ribs below those that do not connect to the sternum at all and are thus called "floating ribs."