I have always thought head coaching salaries in the NFL are surprisingly low

You are now paying $20 - $30 million per year to players who could easily get hurt, and not be available at all, and even when they are available they are only one of 11 men on the field at any given time.

But the head coach touches everything and sets the tone and strategy for the whole team, and they make much less. And they are not even impacted by salary cap. I don't know why the best coaches aren't easily making $20 million per year.