Im not so sure that history shows that is random when you look at the big picture. A few years of randomness doesn’t misplace decades of opposite proof.
Belichick, McVay, Reid, and Tomlin have been in 15 of the last 21 Super Bowl’s.
Before that - Gibbs, Walsh, Parcells and Levy were in 11 of 15 Super Bowls.
Before that - Knoll, Landry, Madden and Shula were in 10 straight Super Bowls.
I don’t see much randomness in at least making a Super Bowl appearance. It seems like, outside of the random lighting in a bottle teams that make up a small percentage of Super Bowl appearances, sustained success coaches make up the majority of Super Bowl appearances and you either have one of those coaches or you don’t.