That's the nail on the head. There is no such thing as a real simulator. Every simulator ever created makes assumptions, assumptions which make simulation easier. You can't account for every variable ever unless you have an exact replica of the entire universe (and more if you consider things to exist outside our universe, which of course they must since our simulator must exist outside our universe. we have ourselves a paradox.)
.....Aaaaaaaanyway. Gran Turismo is no exception to that rule, it assumes things in order to make life easy. For example, where do all these credits come from? they don't, its just an assumption that they exist. How does our car get to the track? its just an code that has been created for our convenience. If it were real, we would have to tow our car to the track. This would be more realistic but less convenient.
In the Gran Turismo world, realism is sacrificed massively to improve convenience, in this respect it will never become real. Mechanical failure is one such inconvenience. Its a huge part of racing, but so is getting sponsorship for your car. Just because it is realistic doesn't mean it should be included. I wouldn't want to play a game where I had to scout round and negotiate with sponsors just so I could race. It wouldn't be fun, its a negative things. Thankfully this so called real driving sim can ignore those annoying issues. I feel this is the exact same case with random mechanical damage, its a negative to your experience and it is something completely out of your control, negatives aren't bad as long as you have some degree of control. Thankfully our real driving simulator can ignore this too and get on with the stuff we enjoy. I would go into an inconsolable rage if my car broke down for no apparent reason in the dying hours of my race at Le Mans. Random mechanical failures are bad, if they could be eliminated from racing they would be. Our simulator can do that thankfully so we don't have to go through the misery of a race driver. Non-random mechanical failures aren't bad, infact I would welcome them. Since you can control them its up to the driver to ensure it doesn't happen. That's another good way to differentiate good drivers from bad ones.
If its between me having a life, and letting Bob do the work for me, I am willing to let him have his rare race to allow me 100% completion.
If you don't care about 100% completion, then you shouldn't have a problem with not being able to do the race, so you don't need to give up your life to do the 24 hour race.