First, most racing engines are rebuild frequently. In the case of drag racing, after several runs (less than 10) the motors of highly sponsored teams get rebuilt. Therefore some valve burning is expected. The valves and heads are specifically designed and treated to tolerate conditions that would burn up a conventional valve in a street car.
You get back pressure at the exhaust manifold. There are sophisticated CAD/CAM programs for designing intake and exhaust manifolds. Back pressure in the exhaust keeps the newly charged unburned fuel from moving through the cylinder into the exhaust system. In racing engines some raw gas in the exhaust is OK as it makes for a more powerful engine and you’ll see flames coming out of the exhaust. There are no pollution controls on these motors. They are tuned for power.
Street cars have all kinds of anti-pollution requirements and drivers don’t want to do valve jobs every few weeks or months. Designing in some back pressure is just one aspect of building the motor to last, get good power, and not pollute beyond legal limits for the life of the car.