The actual X's and O's theory of coaching is relatively easy, though the how often it gets ****ed up suggests to me that the people who end up coaching are rarely the cream of the crop. The exceptions, the cream of the crop who do stick with it, are glaringly obvious - the Belichicks and Sabans of the world, and those guys are all... unique personalities. Others are driven off by the hours and the horribly ****ty pay for everyone but the top 0.1% of the profession, so you're just left with the best of the dregs rising to the top.
I would take issue with the idea that coaching football players is necessarily easy, though. Even in the NFL, it's like being a teacher to 50+ manchildren in their 20s who have been coddled and told they're the best their entire lives (or, for college coaches, an even worse thing - 90 undergraduates, who are basically sub-human as a general rule even when they're not the big men on campus). That's enough to drive most people who could excel out of the profession, and enough to give the rest of the profession premature gray hair. Unless you're Pete Carroll, who just lets his favorite player run the team.