I guess we could also flip it and ask why don't we offer PhDs in developing software for public administration?
I think it might also be something else.
WUSTL has 20,000 students and faculty. This is not a big organization. To manage that, they have over 17,000 administrators. Meanwhile, the Pentagon in 1941, at the start of US involvement in WWII, had only 24,000 civilian employees to manage over a million soldiers.
Some universities will also claim that the average financial contribution for students and families has not increased, in spite of tuition and fees outpacing inflation for half a century or more and student loan debt reaching $1.6T.
But any large bureaucratic organization tends to seek expansion of its staff, budget, and influence, and that is likely a core reason for the dramatic increase in non-teaching university staff.