a. Exhaust comes out of exhaust pipes a lot faster than plane is flying (i.e. it isn't sucked out) For the Spitfire, at top speed, the exhaust thrust was providing the equivalent to 70hp at the crank.
b. In a lot of airplanes, they use sodium filled exhaust valves, to improve the heat transfer from the valve head through the valve stem to the cooling jacket. If you can keep the valve head cool, it doesn't "burn". Manley used to sell sodium cooled valves for Ford FEs. But they cost like airplane parts too.
c. The majority of airplanes use leaded gasoline still. And the lead in the exhaust tends to stick to the valve, coating the valve seats, and so protecting them.
@19Walt93 probably remember when lead was phased out all the concerns among "classic" hotrodders about valve/valve seat issues.
d. Aircraft engines really don't make that much power - Merlin in Spitfire and Mustang only makes 1,730 hp from 1647 in^3 - barely over 1hp/in^3. What they do is make 75% of that power for 500 hours (or more).