I’m thinking this is going to be de-bunked...
Those pilots are usually the only ones flying these things, and the seats are custom made and set to those body types or frames...
Things like g-forces...You need something snug and custom to take the loads on your body when flying such high performance aircraft...
They don’t do it so much in the military because you don’t know who’s going to sign for the aircraft from flight to flight sometimes...
But these racers are a little different breed...
I've raced sports cars. The seats in faster racing classes are also often customized to the driver. Even when you are in a lower budget car, the seat wraps around the driver's sides and holds him in place.
However, I've also experienced a fatigue failure of a seat mount bracket in an older vintage race car. There is nothing as painful as behing held in place in a race car under heavy (high G force) turning or braking by only your seat belts, with the weight of an unsecured seat, with pieces of the broken mounting brackets and framework still attached to the seat loose against your back sliding around with you and pressing you into the belts even harder.
Even among professional teams, seat failures or seat bracket failures have occured, though they are rare. In 1993 I think it was, at Road Atlanta, the Jaguar team was using a very high downforce setup on their GTP cars, and the weak link turned out to be the seat mount brackets. All of their cars retired from the failures, and one driver went to medical with groin injuries from the seat belts when his seat mounting bracket failed under braking.
Engineers usually don't think of seats and seat mountings as being potential points of failure, or being susceptible to fatigue failures. That's a mistake.