Because I don't know any NBA coach in the NBA who hasn't been a former NBA basketball player. Can anyone name one?

It's rare to find any coach who hasn't played at least one game in the NBA. I doubt there's a rule prohiting someone from coaching in the NBA without NBA playing experience, but I'm sure teams would prefer the experience.

It's like professional librarians usually started their library career when they were in high school and shelved library books. In the same way, an NBA coach has paid his dues and gotten experience by playing basketball in high school and college and in the NBA.