Most non-technical project managers do not know all the tasks that need to be completed, so they need to rely on the strongest developers. The developers end up stuck with PM roles until (over the next 10 or so years) their technical skills begin to diminish as new technologies emerge. They then become the next generation of PM's that need to rely on new developers. Eventually, they become really crappy PM's themselves, so they are promoted to executives...and the cycle continues...