Out in the wild, the mom is not always able to take care of them properly and scientifically, and leaves them several hours at a time. Sometimes even a full day.
We had for example a couple of cases where a mom was rescued, but no visible new born kittens. And so they let the mom out again, and she could apparently feed the kittens. Whom apparently survived, and got rescued later. Probably not all, but still enough many to be a rescued litter.
So in a way, I do understand when you say then are a lot toughter.
The problem is, its as in the saying "with such friends they really dont need any enemies". Its no wonder those who survive in such conditions, do have excellent immune system and good healing meat, way above average.
These without do perish, as simply as that.
With home cats, we dont want the bigger half of them to perish, just because we couldnt give them good but really pretty basic normal care.
So this with how tough they are, depends on what you compare with. If enough desperate, we may hope somebody will survive even the most dire disaster.
But normally, we do everything to prevent these dire disasters....
