Turtles and tortoises cannot crawl out of their shells because their spine and ribs are solidly fused to the bones that make up the shell.
shell trivia :
Turtles and tortoises have hard, protective shells that are made up of 59 to 61 bones covered by plates called scutes. Like your bones, a turtle’s shell is part of its skeleton.The shell’s top is called the carapace, and the bottom is the plastron. Turtles can feel pressure and pain through their shells, just as you can feel pressure through your fingernails. Some turtles can pull their heads, legs, and feet inside their shells. They are known as "hidden-necked turtles.” In order to make room inside the shell, they sometimes have to exhale air out of their lungs, which makes a hissing sound. Other turtles can’t pull their legs or heads into their shells. Some of these have long necks and protect their heads by tucking them sideways up against the shell. They are known as "side-necked turtles.”
The leatherback turtle’s Dermochelys coriacea shell is a little different. It does not provide the bony, heavy protection of other turtle shells.Instead,the leatherback turtle’s back is covered with a leathery skin supported by tiny bones. This adaptation allows the turtle to dive up to 3,000 feet (900 meters) below the ocean surface. At this depth, the incredible water pressure would crush a turtle with a heavy shell and less flexible body.