Well the other aspect that people don't really consider is that, with high-end players, the $ just isn't good enough to coach.
Like if Ed Reed wants to be a DC, he'll need to spend several years as an assistant coach most likely before that happens. A good assistant coach probably makes $500K or less. Is Ed Reed really going to work 60-70 hours a week doing coaching, film study, traveling, etc. for 7-8 months a year for $500K? I doubt it. Guy made $51M in his career. He can make $500K off endorsements or marketing, easily, with like 1% of the work/travel.
Like for these high end guys, its not that they probably couldn't do the job. It's that they'll never make the time commitment to it to be as successful as they should be.