Because as a coach, you have what's called a "philosophy." It is a specific scheme that you have studied and mastered over the years, to the point where you know every tiny aspect of what every position is supposed to do and where they are supposed to be in all situations, and feel comfortable teaching that system and executing it. Every coach has a philosophy, and as an owner, that is what you hire. For example, Rex Ryan runs his 3-4 defense, that is his "philosophy." As the owner of the Jets, that is what you are hiring for your team and your players. You can't expect a coach to just come into a team, and all of a sudden be blessed with whatever knowledge of the scheme best fits the players already there. You can't expect Ryan to come in and say hey, I think we have the players to run the Tampa-2. Ryan doesn't know the Tampa-2, therefore he can't teach it to his players. That would be like you going to school to be an engineer, and when you start looking for a job, your wife says, "Well there are tons financial advisor jobs, why can't you take one of those."