I'm an amateur racer, and I can tell you that seat belts are only a small part of the equation that makes racing crashes survivable. Racing seats are built to fit individual drivers, for instance, and the cocoon -- the portion of the frame where the driver sits -- is designed to protect the seat.
Additionally, race cars are designed to essentially disintegrate in hard wrecks -- the cocoon stays intact, but the rest breaks away as a means of absorbing energy. Road cars have a similar design with crumple zones, but race cars take it many steps further.
It's also come to light in the last ~15 years that belts are simply not enough in some instances. Dale Earnhardt (possibly the most famous racer in history) died in an average-looking wreck at Daytona in 2001 because his neck snapped in a collision with the wall; drivers are now required to wear head and neck restraints to prevent this.
Track design plays a role as well. You've probably seen tire walls and gravel traps, but newer technologies -- like SAFER barriers (energy-absorbing walls made of steel and foam) -- have come along and are replacing the old guard.
There are certainly a lot of other things in racing that contribute to safety. But the biggie is always the harness.
A roll cage won't do jack for you if you're flying around in the cockpit smashing into the roll cage. Crumple zones are also ineffective if you're not strapped in.
Race cars don't have air bags. If you're strapped in tightly, the air bag adds nothing.
I've been in a crash where the belt saved my life, or at least prevented a crippling injury. I walked away with a broken nose. A roll cage or a HANS without belts would not have helped.
One of my cars has three point seat belts in front that do not spool. It is infuriating to unlatch the seat belt every time I have to back-up the car or reach for something on the passenger side. Racing harnesses are even more restrictive when adjusted properly.
Additionally, race cars are designed to essentially disintegrate in hard wrecks -- the cocoon stays intact, but the rest breaks away as a means of absorbing energy. Road cars have a similar design with crumple zones, but race cars take it many steps further.
It's also come to light in the last ~15 years that belts are simply not enough in some instances. Dale Earnhardt (possibly the most famous racer in history) died in an average-looking wreck at Daytona in 2001 because his neck snapped in a collision with the wall; drivers are now required to wear head and neck restraints to prevent this.
Track design plays a role as well. You've probably seen tire walls and gravel traps, but newer technologies -- like SAFER barriers (energy-absorbing walls made of steel and foam) -- have come along and are replacing the old guard.