All I can say is that the belts we remove every 3 years are not oil stained or have fuel marks (what would a fuel line do inside the drivers compartment?) leaking seals would not effect the belts as a properly built race car will not allow oil or trans fluid get to the driver.
sealed driver compartments are a myth. also covered motors with a hood to fire wall and all. Offroad is the worst for both of these.
poke your head inside a sprint car next time ya see one, ya straddle the drive line, ya have wing hydraulics , fuel lines, your knees are against and straddle the steering below there ya got the drive line and fuel pump, oil and steering lines. No way to keep the belts out of that. And it all goes backwards. I personally have had a fuel pump start leaking. I can tell ya it don't take long to soak ya with fuel.
the lines blow, seals start leaking, every dirt track race I go to I see at least one smoker or worse a night. Not all race cars belts stay in the rooster pit. lots have at least one that runs into the trans tunnel.
Another thing to add to 5 racers post is seat belt mounts and angles, most are not mounted well, as are seats. Seats break and belts are of no use then. composite seats with no back support I have a hard time beliving they don't break, I know there tested and all. mine has a inch gap and it bothers me.
I don't think I have ever seen fused battery cables on anything yet. Disconnects 5 or more feet away is about it.
I also like looking at the old race cars and think, wow that all worked, and those welds held. Then again dad raced sprint cars without cages and says of sprint car drivers now days, take there cages off and see how big there balls are.