Kim did stay with the vehicle (and his family) for something like a week before going for help. I think this might have been the right decision at that point, because his youngest child was 7 months old and would not have been able to survive without food nearly as long as an adult would.

Kim's wife and children did stay with the car and were rescued some time later. But at the point that Kim left the car he had to be thinking that searchers weren't going to find them in time.

I've often wondered if Kim's fatal mistake wasn't leaving for help but rather leaving the road he was on and going cross country. By the time he made the decision to do that, his thinking might have been very seriously impaired indeed, though.