If there is no economic motivation, the incentive for coaching often leads to oddly motivated individuals and less tangible rewards like power, control, or fulfillment of some weird personal egos. You have to spend hundreds of hours as effectively a volunteer.
Of course, some fantastic folks are doing it, too. But the *profession* has also attracted a lot of folks who shouldn't be allowed around kids due to some odd alignment of motivations. The people most qualified to coach high school sports are very, very rarely the ones that do it.