Eggs can crack inside the hen, but it’s not common because eggs are actually pretty strong when forces are applied evenly over the entire shell.
If you wrap your whole hand around an egg trying to make as much surface contact as possible, it’s actually pretty difficult to break the egg by squeezing it. If you pinch the egg between two fingertips, it’s much easier because the force is concentrated on a smaller area of the shell.
Imagine pushing an egg on to the point of a sharpened pencil. It would pierce through pretty easily, right? Now imagine pushing an egg onto a soft ball of clay. It takes much more pressure to break it. So you can imagine if the insides of a hen are pushing on the egg evenly from all directions, it gets even more difficult.