I have a theory...
Because there's a difference between being a good head coach and being able to TEACH someone to be a good head coach.
In the late 90s/early 2000s, when he medically retired from WWF/WWE, Shawn Michaels opened a wrestling school. One of the biggest criticisms of him as a teacher (most famously from Bryan Danielson) was that while he was undoubtedly one of the best wrestlers of his era (and possibly of all time), he had a real problem teaching others to do what came naturally to him.
It's possible that Belichick is the same way.
Oh, you mean basically the whole 2020 NFL Draft?