what is school's incentive to have PhD programs?

I've been haunted by this problem for a long time since I decided to enter academic field. For non-science PhD program, accounting for instance, what is the incentive for a school to have it? They pay you a lot of money (5rys tuition plus stipend) and train you. Faculty spends a lot of time on you. Maybe you can do some TA or RA. But in the eye of faculty, those work is trivial compare to their spending. Moreover, in top schools, those obligations are even limited by fellowships. When you are done, you go to other schools, which is competitor for yourAlma Mater?

First of all, competition is also a means of cooperation in academia.

Seconly, I think one of the missions for top business school PhD program is to "produce" future faculty in research universities which obviously have PhD program, while the one for lower tier schools is to "produce" future faculty in teaching schools. It is true in one sense.

While I talked to one professor at my current school, he said, "Every PhD program wants to do some research in the area it has strength. Obviously, faculty don't have that much time to explore every corner of this area, and then the task is on the PhD students' side".

one of the most important things for a PhD program is its product, namely the phd graduates. the placement reputation and academic infulence of alumni are the driver for their investment on phd students.

I do not think that faculty will recruit phd students only for RA or TA. One professor once told me that those work depends on your own need. Accumulating research exqerience and enhancing presentation skills are much more important than earning one's life.