Ribs have an important role to play: they protect the internal organs of the chest from injury. They wouldn't be very good at it if something as minor as a break was likely to allow a rib to poke and puncture things.
The ribs act as a unit (they all move at the same time) and they offer a lot of protection to the heart, lungs, etc., because they are attached to each other and the muscles around them.
With a broken rib there is no practical way to immobilize it... This means that it could poke your lungs causing you to have a collapsed lung. It could also poke your heart... [etc.]
Look at the diagrams below, and imagine a fractured rib. A broken rib is not wild canon likely to do any of the above.
Note carefully the amazing amounts of connective tissues (fascia, muscles, tendons, and ligaments) that attach to the "rib cage", stabilizing it.
The ribs are stabilized externally by groups of muscle. These attach to tough fascia surrounding the ribs. Between the ribs are three separate groups of muscles running in three different directions (diagonally back to front; vertically top to bottom, and diagonally front to back) attaching securely each (section of each) rib to the one above and below it. Finally they are attached to the spine and the breastbone.
This is why rib fractures heal without splinting. The ribs aren't going anywhere; they basically stay put even when they are fractured.
The vast majority of rib fractures are non-displaced, followed by minimally displaced ribs. These heal in place (even with continued movement and activity) through the usual process of callus formation, because there's nowhere for the ribs to go. Eventually the callus becomes big enough to contain both fractured ends of the rib and reunion takes place.
Rib fractures might cause pheumothorax or hemothorax if the rib is struck with great and narrowly focused force: the end of a baseball bat swung full force, or being thrown (rarely simply falling) against an immovable projecting object. Even then, in most cases the rib is not repaired; if it is projecting into the pleural space, it is manipulated to be less so, and kept under observation until healed or no longer a danger.
Under certain circumstances, rib fractures must be surgically repaired; this is when enough multiple adjacent ribs are fractured at multiple places so that that a significant segment of the rib cage no longer functions as the rest of it does; this is called a flail chest. Since respiration is compromised, this has to be repaired.
One or two fractured ribs are rarely a cause for concern (as opposed to what caused them). Multiple rib fractures, especially adjacent, indicate greater level of trauma and must be treated differently because of underlying injury or the high risk of developing pneumonia or other complications:
The greater the number of fractured ribs, the higher the mortality and morbidity rates. Patients with isolated rib fractures should be hospitalized if the number of fractured ribs is three or more. We also advocate that elderly patients with six or more fractured ribs should be treated in intensive care units due to high morbidity and mortality.
Rib Fracture Repair: Indications, Technical Issues, and Future Directions
A comprehensive analysis of traumatic rib fractures: morbidity, mortality and management