Actually, they don’t. The military cut back their training severely after one of the last Base Realignment and Closure exercises, consolidating some training that had been spread out to several services into single bases and closing a number of trading bases altogether. They then gave up the airspace and the bases. That both cut back their training and severely limited their ability to increase it. Same for the reserves and Guard. As a result they developed a shortage which they managed by both increasing the active duty service commitment and increasing retention bonuses.
Where traditionally, they trained a lot of people, half of whom left at their eight year point, they now train much fewer who are obligated for ten years and then bribed into staying until they make 20. So the airlines are getting fewer and older ex-military, at least fixed wing military.

And what reserves are left are increasingly mobilized for deployments making even the reservists less available.

So no, the majors CANNOT fill their slots with retiring military.

Granted, RIGHT NOW the shortage is at the regionals, and the majors NEVER WILL be as short as the regionals, given that they pay four times as much for basically the same job, but this is unequivocally a time of real opportunity for regional pilots, and it’s going to keep getting better for another five or six years.