Why don't any MMA fighters sprawl to defend a shot?

They do but a lot really depends on some other things. First you have to remember that it is fighting not wrestling and so a fighter may be somewhat reluctant to sprawl and instead want to use double under-hooks so that they can use their knees and knee a person or maneuver a person to where they are more vulnerable and then unload on them. Also sprawling requires room and if you are up against the cage you are not sprawling anywhere. The cage up against your back side will not allow you to do it and so using things like double under-hooks and turning them or once again maneuvering them into a position where they are more vulnerable is really more desirable.