Do the math: The number of professorial positions is not growing (or not very quickly). Under steady-state, each professor only needs to train one single tenure-track PhD in their entire career. (Or maybe 10 professors need to train 11-12 students, to account for various forms of attrition.)

If you want more PhD's than that, you have to figure out a place for them to work; industry is a good "pressure relief valve".

With the aging population, we also hit peak college attendance 15 years ago.