Same as when Trump issued his no-trans-in-the-military order his first term. The Pentagon resisted. They had not asked for the ban, and they didn't want a ban. Why would they want to get rid of people they'd spent treasures in time and money training?

I'm sure Trump doesn't get it, not ever having spent any time in the military, but it's not hard to understand if somebody explains it. The military already enforces a pretty strict "up or out" personnel policy. Officers and senior enlisted people can't extend their terms of service unless they get promoted on or ahead of schedule.

This is meant to avoid the Peter principle, where people rise to their level of incompetence. So the average or incompetent in the military can't keep serving. The career people who do get promoted have excelled in their career fields and completed years of intensive military training in those fields. They've continued their education with university or postgraduate work (typically at night or on their own time), and they've impressed their superiors as skilled leaders / managers.

Nobody wants to throw all that away, least of all a professional manager who would be aghast at the very notion. If anybody gets that, senior military managers get it. They've had it drilled into their heads from the time they were second lieutenants that their most important job is taking care of their people.