Naive question: why can't they just have more tenure positions?
MIT graduates about 400 PhDs per year and only has about 40-50 tenured positions open each year.
I suspect every other program is similar. There are simply too many PhDs being graduated, but the universities are dependent upon them for slave labor.
Not mentioned yet is the change from defined benefit pensions. If you had a final-salary pension, then there was no financial incentive not to go emeritus, in the good old days. Whereas on a defined-contribution scheme, retiring a year earlier does literally cost you (or your estate) a year's salary.