I am not sure that the facts lead to the conclusion....
Big-time schools get their pick of established head coaches - and they tend to do well because, naturally, they are at big-time schools. Less established SEC programs have to roll the dice with coordinators, and their chance of success is lower because they are coaching at the have-not programs.
The fact that the combined record is just under .500 means that first-time coaches are not total failures, just that they are not rousing successes. If you hire someone that can coach but just needs a chance, a first-time HC can work.